Almost helpful to have the full banal case for the relevance of 'non-binary' as a concept set out like this. You could have added '#bekind' at the end, just for completeness.
So to break it down bit by bit:
The question would be why does it matter to you?
The impact the categories of male and female have on society as a whole are well explained by others on this thread, not to mention years of feminism.
''Non-binary'' also probably means different things to different individuals."
Quite. Which is one of the main problems with it, when it's seeking special status in law and society.
Some people will use the term because they feel they don't conform to stereotypes/can't relate to what society associates with being a man or a woman.
Or they want to present as androgyne, or they feel they have both male and females qualities and can't quite fit in one ''category''.
And, do you not think, it would make more sense, for our collective, societal response to be to people who feel this way, to be, 'We need to get more comfortable with expanding what we understand to be "male and female qualities"' rather than, as you're advocating, 'oooh yeah you're right, you don't fit. Better make up a new category!'
Because that's the problem with your - I'm sure well-meaning and commonly-held - viewpoint. Amplifying the neoliberal battlecry of 'what I do is none of anyone else's business', we reduce the potential of us - as a collective, as a society - to evolve and develop in a way that makes it better for everyone.