Why revisions table is necessary?

I think I have better understanding of fs now but I want to
make sure I really understand big picture before I start any
coding.

Here are my understanding of fs revisions:
1. Filesystem revision apply to whole fs, not to a file or to
   a part of tree
2. Every commit bubbles up to root node

If #2 is true, every fs revisions correspond to root node
revision ID. Unless there are braches, fs revision 1
corresponds to rnode ID 0.1, 2 to 0.2 and so on. Even if
there are brahces, you can map fs revision 1.1 to root node
revision ID 0.1.1 if you loosen the rule of fs revision to
be integers. So why is it necessary to keep separate table
to manage fs revisions? I don't see why fs revision have
proplist since root node already has one.

Or can fs revisions be applied to part of tree and revisions
table is to record the root of that part?

I think I'm missing something obvious, but I don't know what
it is... I hope I'm not asking a dumb question.

-- 
Yoshiki Hayashi