In software development, you often need to know: when was this feature released? in which releases was this changeset included?
Typical example is: you're debugging a problem, you find out what's the changeset introducing it, now you want to know when that changeset was firstly released (i.e. included in a tag) - how to do that?
hg log -r "cset:: and tag()"
where cset is the changeset you're looking for.
So know I know that Sphinx new way to load searchindex.js was introduced in the this changeset, and released for the first time in 1.0b2:
$ hg log -r "423faa03c908:: and tag()"
user: Georg Brandl <email@example.com>
date: Sun May 30 19:52:08 2010 +0200
summary: Release preparation for 1.0b2.
PS: why did I need to know that? because Matplotlib is still using the old way, but Debian needs the new one.