Several things, really, but the main ones are these.
I get to tell the story that has been rolling around in my head for years. I'm finally getting it out of my head and putting it down on paper, finally giving it some kind of definite form and no longer have to worry about what I should be doing with it - it's already in the process of getting done.
And other people are reading it. It's nice to have a lot of readers - and it helps when I try to support my comics-creation through Patreon and such - but it's not super-important. As long as someone is reading it, and caring enough to comment or share or something, that's good enough. Some of my favourite moments of creating Grassblades has been when people I don't know IRL have told me that they're looking forward to the next page, or that they really care about what's happening to the characters.
Stories are meant to be told, and they aren't complete until somebody reads them.