Ex: Andy Pettite, Roy Halladay, Edwin Jackson, Ivan Nova, Cole Hamels. Is it just a habit or does it actually have benefits?
It’s simply a mechanism for establishing a rhythm or tempo. It works for some pitchers like those in your list. And those are some pretty darn good pitchers.
Ah, yes—the old-fashioned full windup. I used it most of the time and only went to the stretch when there was a runner, or runners, on base (except when the bases were loaded, in which case I went back to the full windup). Yes, it could be a timing mechanism, but I never thought about it that way; I just did it, and I had good results with it.
The whole point is to work fast—which I did, which most major league pitchers who used that windup did. 8)