That evil spectre of marketing: it hangs over every web post. This is a morally clean, simple, non-douchey way to allegedly increase traffic.
“Long Tail Keywords”
If you tag posts with multi-word phrases then they are more likely to rise to the top if people type that phrase when searching. Being more specific helps to increase your ranking. This was new to me, but there you go. There are other tips at that page.
I came up with a few for this page (at the bottom).