I do for a couple reasons.
1) Like others have pointed out, to make sure things are making some modicum of sense (even if it takes forever for something to pay off XD) Continuity. And refreshers so I don't forget I was going somewhere with something dropped early or midway in the story.
2) To edit. You'd be surprised how many times you can read something and still miss an error.
3) I uh...I kinda started writing this comic for myself anyway. I'm not saying it good, or that I love my own work, I'm just saying I am writing this story for myself to entertain myself and want to see it through to the end of the story. So...reading it myself is kinda the primary target XD I like to share it with others, and am so happy other people are enjoying the ride with me. But even before posting this publically, I've been working on this for myself.
I hope that doesn't sound arrogant. I am not claiming to be anything I'm not. I just have these characters I love, and a story I want to see to the end. Growing up in the country before Internet really took off (didn't get it until High School), I had a lot of time on my hands to be "creative" (not to say I didn't work, or play, or do other things). I guess I have always wrote stories for myself to entertain myself XD