Thats a good question, and something that baffled me for quite some time. Having looked into this, it appears there are a number of reasons for the non-binary transgender identification:
Non-binary is now much more popular with teens (as my teens and their friends tell it, plain trans is not that cool anymore). But they firmly understand this to be a trans identity, because they've all been taught that ever-expanding trans umbrella.
Non-binary requires no changes whatsoever, which far more sections of society accept than when one identifies as the other sex, whom most people still assume and therefore expect to be transsexual individuals who have medically transitioned all the way to full genital surgery. But with non-binary, there is no expectation as to expression, behaviour, hormone treatments or surgery.
In my view, this is a positive step as no one should ever feel they have to medically transition. If identifying as non-binary allows them to reject the sex stereotypes and sex role stereotypes society imposes on someone of their sex without feeling the need to make irreversible changes to their bodies, that is a good thing. (This is currently happening with a lot of young women and older girls, often bisexual or lesbian, who seek to escape the pressure society puts on women as to how they should look and behave.)
Non-binary plus trans is also a common path away from identifying fully as the other sex. This group is growing at the moment. Maybe it is a safer way to detransition as it avoids the hostile reactions many detransitioners receive from their own community. They haven't really detransitioner after all, they're still under the umbrella. But it's a good way to step back from the medical path.
Then there's the males who identify as non-binary and the other sex without ever having medically transitioned or identified purely as the other sex (who seem to be far more common than females who do so). The individuals that come to mind appear to want access to women's spaces while going about their lives virtually unchanged.
There's also a sizeable group of various ages that do it for attention and effect.
Others do it to be able to claim the protection they believe a trans identity entitles you to. There is currently nothing in the law that would protect a non-binary employee from being sacked for not conforming to say, strict dress codes at work for women, but there is for those who are even just proposing to undergo gender reassignment. I'm certain this has not yet been tested, but there is a reasonable assumption that there is no protection under the law unless one also claims a trans identity.
My view is that I don't care how people identify or dress or what gender-conforming or non-conformimg things they do. I simply do not believe that our minds and our bodies are two distinct entities that can be mismatched. That kind of metaphysical, quasi-religious belief is something others may adhere to if they wish and I respect their personal choice to do so, but as with my very religious Christian friends, I expect to have my right respected not to share this belief in transgender ideology and that I am not expected to suppress my disbelief or worse still forced to espouse a belief I do not share.