What is the apt cache good for?

[Hendrik Boom <hendrik@topoi.pooq.com>]

I can get some space back by doing apt-get autoclean.

I can get more back by apt-get clean.

This deletes lots and lots of files from my system.  Judging from du's 
output, /var/cache/apt/ takes a bit more than a third of my disk space.

Now I've been keeping these files around, just in case.

But do I actually need any of these files?  Or are they only needed while 
I'm actually busy installing stuff?  After those packages have been 
installed, are the corresponding files in the cache good for anything at 
all?  Perhaps for deleting the packages later?

(By the way, I use aptitude)

-- hendrik