What is notmuch compact and should I use it?

[Attic Hermit]

I read the manuals of Notmuch and encountered notmuch-compact. The
manual says that it compacts the notmuch database and it will enhance
the performance of Notmuch.

What exactly is to compact the database? I noticed that the size of the
notmuch database is shrunk after `notmuch compact`, but I don't think
this procedure is gzipping the database. I thought it is something like
the garbage collecting, but I'm not sure.

And should I use it regularly? if it is, how often should it be done?
-- 
Attic Hermit