Original article: http://www.blue-bag.com/blog/excluding-common-requests-your-apache-logs
Log files can get filled up with repeated calls to files such as favicon, robots.txt, images, css js etc
Mostly you want to log the initial request for a page and not all of the resources subsequently requested.
Troubleshooting other issues may mean you would log files such as favicon, images etc - but generally they needlessly fill up your logs.
Do it ocassionally to look for missing images etc.