mod_rewrite is actually a pretty big subject
and quite difficult to just sum up like that ... at least for me.
Basically mod rewrite good ..
do it ... do it ...
It means more keywords in your url ... which google likes to see ...
Its more human readable as well ... take a look at
Safari Club - African Safaris for example .. now wihtout mod rewrite it'd all be index.php?id=22 blah blah blah ...
I feel tired and I'm probably not explaining it all too well