ANSWER : Because weather I like them or not rewrites are an import part of the creative process. Many people that don't write misunderstand the term rewrite as to mean a COMPLETE rewrite from the beginning and for me (personally) a rewrite could be as simple as changing a character's name, who they are in the story or changing a few scenes, yet not the whole script. Rewrites help writers and directors get their stories hashed out and hopefully provide the best film possible for their fan base.
ANSWER : It all depends on what I'm rewriting, if it's a character's name I look at the script and make note of every time the character speaks or it mentioned in the script and make the changes accordingly. If it's changing character's death scene (I do this often until I find something I really like) I look at the prior deaths in the movie and try to build on them in intensity etc.
ANSWER : As many as it takes to get the script the best it can possibly be. I rewrote the script for Wicked Carnival 15 times before the final draft was good enough to turn over to readers. Sins of Man : Rise of Mortis was rewritten 12 times so I feel there is no "magic number" of rewrites.
ANSWER : This one is easy speaking form experience I would day for me the worst time to have to do a rewrite is on set in the middle of shooting, it's time consuming when you're trying to direct a scene, your in scenes that day and you have to rewrite dialog due to a cast member not showing up on the last day of shooting .
ANSWER : I laughed for like 5 minutes when I read this question and it was the most often asked one, I was like WTF !?!?! Rewrites are impossible to avoid, they are a part of the job and will happen even on the biggest films by the best writers on the planet. As much as I dislike rewrites because I try very hard to get the details right the first time I do have to rewrite and rewrite and rewrite again until I get it right and the flow of the story is perfect.
update on sins of man ii : vengeance !
STAY HUMBLE, STAY HUNGRY MY FRIENDS :)