Not a specialist, but I suppose it has to do with having different configurations for different top level folder. In Unix-like systems, every top level folder have a different purpose, and what works for the root may not for /tmp, /swap, etc.
In those example, no need to snapshot /tmp, as it is a forder whose file are bound to be deleted, and for which being able to restore has no use.
/swap is pretty similar , and is often formated with a dedicated filesystem.
/usr often only change during the package manager transactions, so snapshots are often tied to that, while /home may be set to keep daily snapshots.