10) It helps support the market for red pens.
9) You can test-drive a few adverbs without getting yelled at.
7) It provides amusement for your critique partners.
6) You can use lame jokes, stereotypes, bad grammar, and stilted dialogue, knowing (hoping?) they'll disappear during the revision process.
5) It's a good way to use up scratch paper.
4) You don't have to show it to your agent, editor, or spouse.
3) It makes good kindling.
2) Your dog/cat/gerbil thinks it's terrific, no matter how many words are misspelled.
And the number one best thing about a first draft:
1) There's only one place to go from there: Up.
(see this post for a previous rumination about first drafts)