I notice that Nulls have a collapse transform switch. Just trying to imagine when this would be useful. Something related to the nifty "Scale Z" precomp trick, maybe?
No purpose. At least none that I have ever found. Null's don't render so they should not effect calculations.
Not anything at all. Nulls are just a hacky solids without renderable content. Was probably too much trouble to implement a completely new layer type at the time back then...