I actually have the same question. I know you're agender so I was wondering why you personally wanted to wear a binder.
Let me start this by apologizing in advance for any snark you feel is unwarranted in my answer. It’s not necessarily directed at you, just the dozen-plus Asks I got asking the same thing with varying degrees of rudeness. So, on that note:
As an agender person, why wouldn’t I wear a binder? Or a dress? Or both, if I felt like it? By the sheer definition of the word, I don’t apply gender roles to my identity and that extends to clothing, as well. That doesn’t mean I walk around in a potato sack, it means I wear whatever I want, when I want.
A binder is a handy way to present as even less gendered, and it’s especially handy when I’m wearing a stiff cotton shirt, the fabric of which tends to chafe my nipples. I’d apologize if that seems too personal, but let’s all be honest: people are asking about my undergarments. I have nipples that chafe easily. Bras have this annoying way of contouring, lifting, and padding in ways I don’t find comfortable or want to present.
Being agender doesn’t mean I have to present as neutral as possible. Yes, I tend to like utilitarian, unisex clothing, but sometimes I like to wear a dress (hell, even a wig now and then), and other times, I like to wear combat pants and a leather jacket, binder ‘n’ all. These days don’t take away from my identity, they just add to it. I’d even go so far as to say, days like that wouldn’t alter anyone else’s identities, either.
Presentation ≠ The Be-All-End-All of Gender Identity; it’s just a fraction of it, the importance of which varies from person to person.
And for the record: it is pretty rude to ask people about their fucking underwear. I’m only answering because so, so many people keep asking and I’m giving everyone a benevolent fucking educational moment.
So, to more simply answer the question, "why do [I] wear a binder?" None of your god damned business, that’s why.