Question concerning depth

I was reading a tutorial that explained how to create dynamicly 4 buttons (well actually its more movieClips that acts like buttons) and the depth of the movieClips created was 100 and 101 etc… A few lines down i saw these lines of code:

_root.createEmptyMovieClip(“imageHolder1”, 200);
_root.createEmptyMovieClip(“imageHolder2”, 201);

Why dont they use level 3 or 4 why is it such big numbers? am i missing something or is it just a way to be sure not to use the same levels 2 times?