In short, so your work won't be trash.
Some people say they're able to edit their own stuff. Maybe that works and yay for them. But I've read books that were self-edited, or they should fire somebody! Sometimes I can't even finish the book because I'm stumbling over grammar mistakes and inconsistencies.
Apparently, I really like the word and! I'm also a huge fan of adverbs. I never have enough description or dialogue tags. I'm forever being asked, "What's it smell like? What's it feel like? What's it taste like?"
Readers will forgive a lot - they won't forgive consistent grammar issues. You've only got one shot at a first impression. Wouldn't you want to make the best impression you can?
Lucia Sugafield, learning how to write better.