Your typical server log directory looks something like this:
access.log access.log.1 access.log.1.gz access.log.2.gz
Now what if you want to extract data from day x to day y? You could use gzip and grep to uncompress the files, but there's a better way: the z commands.
All you have to do is:
zgrep "2010" /var/log/apache2/access.log*
More info on the subject can be found here: http://www.thegeekstuff.com/2009/05/zcat-zless-zgrep-zdiff-zcmp-zmore-gzip-file-operations-on-the-compressed-files/