Usually, you can. But occasionally you get mildly defective tools that require some directory to exist, even though it's empty. It's easier to add a gitkeep than fix them.
You can. But this makes intent clear. If you clone a git repo and see build/ with only a gitkeep, you are safe to bet your life savings on that being the compiled assets dir.
There may be other directories. I think it's useful to be able to see the entire directory structure of a repo when you check it out, and not just after running some scripts.