Many Christians effectively operate with two categories of issue in their mind. First, they have ‘primary’ or ‘gospel’ issues as top tier matters. These are the issues whereby, if you stand on the wrong side of them (as they judge it), you cannot credibly be called a Christian of any sort. You are outside the faith and do not belong to the kingdom. After that, they simply call everything else ‘secondary’. I don’t think they intend to convey that secondary matters are altogether unimportant, though they certainly think some of them are. But they do suggest they are all things we can rub along together over, not worrying too much if we significantly disagree.
The benefit of having only a primary/secondary distinction is it makes it much easier to be catholic (in the small ‘c’, ‘universal’, unifying sense). If all we have is gospel essentials, it is very easy to have a quite literal broad church. Of course, there may be some arguments about what constitutes a ‘gospel essential’. But once we’ve nailed down our handful of absolutely non-negotiable, faith-denying doctrines, that’s that. It is largely the approach taken by some mission organisations, focusing as they do on evangelisation and gospel outreach, and other groups looking to encourage gospel fellowship and evangelical unity.
The problem with the primary/secondary distinction is it fails to credibly help the local church. Sure, most evangelical churches aren’t going to want to admit somebody who openly denies the gospel and call them a believer. We want to hold onto that first-order, gospel issue tier. But we can’t then relegate everything to a secondary matter. Inevitably, there are issues in the church whereby a) we cannot hold diametrically opposed views together because they’re mutually exclusive and b) there are issues that do not write somebody out of the kingdom but make it very difficult to accommodate in particular churches.
As churches, we have to decide what we’re going to do somewhere. Are we prepared to welcome unbaptised people or not? I don’t know any evangelicals who would want to make baptism a gospel issue, inasmuch as we all recognise the thief on the cross went to paradise with Jesus without being baptised. But can a church really just say this is secondary and welcome the unbaptised, those “baptised” invalidly and those baptised in whatever mode they happen to have done so? You may say ‘yes’. But the point is, you cannot accommodate that position and the one that says ‘absolutely not!’ Those positions are mutually exclusive and, at some point, you have to decide what you will do.
Similarly, not only are some positions mutually exclusive, but once you have taken a position you cannot accommodate everyone who thinks other. Take the issue of headship. I want to park the rights and wrongs of either position for now. I don’t think I know of any church that would consider it a primary, gospel issue. Namely, you can take either position on this and still have genuine, saving faith in Jesus. But it is equally true you can’t simultaneously have exclusively male eldership and include female elders. You can’t have men and women preaching in the main gathering whilst affirming preaching in the main gathering of the church us for men. Those positions are mutually exclusive. If you take an egalitarian position on this, you can’t then accommodate in your church the person who believes eldership should be male-only. Similarly, you can’t take a complementarian position and then accommodate a woman who feels absolutely certain she is being led to preach and become a pastor. Those are going to (minimally) cause friction.
For this reason, we need a middle tier of issue. We should rightly maintain a first-order, gospel issue tier. These are issues on which we must consider you to be outside the faith. Things like denying the deity of Jesus or believing in works for salvation would be among them. Then we should have our third-tier, non-essential category. These are things that, whilst still maybe important and have ramifications, are matters that can be differed on and easily accommodated within a single local church. You might think of millennial views, political views, social views and a host of other things. In between these two tiers, we should make room for a second-tier of church issues. These are not issues that are so significant that they will keep you out of the faith, but they are significant enough to make it difficult to sit within the same church. They are issues that allow us to recognise other Christians as genuine believers, whilst not being able to welcome them within our particular local church. Issues such as baptism, expression of charismatic gifts, headship, biblical infallibility and others might all sit within this category.
We all need to work out how we can view others. We will inevitably be ask, can we join together in a local act of witness with Muslims in our area? Why or why not? Can we join in an outreach together with the local Catholic Church? Why or why not? Can we have the local Catholic priest come and preach in our church? What about a local Anglican vicar? Or a Presbyterian? Can we necessarily say yes to that Baptist? What do we do with this couple who want to join the church but don’t believe the Bible is truthful in all it proclaims? What about that guy who believes most of what we do but refuses to be baptised? Can we welcome a person who is definitely a believer but insists on expressing charismatic gifts (in the form they judge it) in ways we don’t think are entirely biblical?
The issue isn’t how you parse these questions. You may well fall differently on some of them to me. In many of them, it probably comes with a bit of an ‘it depends’. But if all we have is primary/secondary, we are left without much help on some of these things. If we have room for a set of issues that do not determine your standing before the Lord, but are nevertheless significant issues for the local church, we may just have a better stab at answering them.

Yes. The first tier issues relate to the nature of saving faith, and what is essential, e.g. the divinity and resurrection of Jesus, and other gospel issues such as what is and isn’t sinful.
The second tier issues relate to the practice of that faith in the local church, e.g. forms and acceptability of baptism, male-only leadership/eldership or egalitarian practice, limits of ecumenical or inter-faith connections. A mis-match between an individual’s belief and the practice of a local church on a second-tier issue wouldn’t affect his or her salvation, but it would make continued membership of that church very awkward if not impossible (as I know from experience).
The third tier issues affect neither salvation nor practice, so can (hopefully!) be ones that people agree to differ on e.g. pre/post/amillennial views. There may be debate on some issues on the borderlines between the tiers, but the framework of them (gospel faith/church practice/views which affect neither of the first two) is valid and useful.