The TITLE tag is one thing that the engines look at to index you, and has been used for some of those bizarre results that people like to play with. (Things like "dumbass" in the title, for a page about your former spouse, or boss, perhaps.) The page title doesn't necessarily "show" on the page, so you can put "topic keys" in that may help the engines to know what you offer.

People have tried "loading" the title with words or phrases that people are "likely to look for," often stuff unrelated to their own content, to try to improve the odds of being "hit;" but Google (especially) does appear to discard title keywords that don't relate to your subject content - sometimes. There's even rumor that they keep a "blacklist" of sites that have tried to fool them, but I'm not convinced it's true. Maybe for people who've tried really hard(?).

John