Every now and again you might want your website to display different information to visitors who arrive at your site from specific URLs. You might want to tell visitors from Digg to digg you again or maybe you want to give a big welcome to visitors from Google and a big kick up the ass to visitors from Yahoo! (not that I have anything against Yahoo!). You might even want to serve different reciprocal links and ads to visitors from different sites.
Tailoring content to different visitors is easier than you might think.