1) Rather than stop and start over again and again, when you allow yourself to write a truly awful, lousy first draft from beginning to end, you actually finish a draft all the way through.
2) Until you write the end, you do not have a clear grasp of what comes earlier.
3) You accomplish what you set out to do.
4) Once you have a skeleton in place, a writer is able to stand back and "see" her story in an entirely new light
5) One of the greatest benefits of writing a truly awful, lousy 1st draft is that it's all up from there...
For more on the benefits of writing a truly awful, completely lousy 1st draft:
The End Is the Beginning
First Draft Twitters
First Draft versus Rewrites
First Draft Blues