I've never liked to revise, or even proofread, for most of my life. I'm not sure why. When I clean and think I am done, I take a last look to catch anything I missed. If I'm eating a good meal, you'd better be sure I check the pot before giving up my plate. Revising, however, I hated. I don't know why. Perhaps it was admitting I had been wrong, or that I could have done it better, though you'd think I was used to that. Maybe it just felt like too much work. It could be that I was scared to see how bad it was. Whatever the reason, When I threw down my pen and pages, I was done, for better or worse. I promise you, it was worse.
Revision, as detestable as it might be to some, is critical to good writing. I don't know why it's impossible to do it right the first time, but it is. I've never ever heard of a successful writer that didn't revise. Moreover, every time I recall an author talking about revision, they talked about how MUCH they revise! Many authors are still mentally revising their work after it's been published.
So, I can't tell you why we need to do it, but I can tell you how it helps. The most obvious is basic proofreading. People make mistakes, all the time. Proofing lets you spot and fix those mistakes before someone else does. Revision allows you to see the places in your story that need tightening, or loosening, or more explanation, or less exposition. It lets you follow the voice of your characters, and better recognize when they shift, than when you were writing mad about the wet newspaper and it soaked into your characters.
I think Writing and Reading are two different parts of the brain, perhaps editing is as well. When you take a second look at what you've written, after letting it cool for a time, you see it as a reader instead of the person writing it, and that makes all the difference. Writers have no clue when they have screwed up. Readers can always smell it. Fortunately, the best writers are both.