The slow pace of the games will build your patience skills. If you're like me and you 'zip' through a couple hundred hands an hour on Turbo Texas Hold'em, you'll find waiting for your computer to respound a new experience. For poker newbies, the games are good practice for learning the mechanical aspects of poker. I think that giving beginners a place they can learn the game without feeling embarassed when they make stupid plays will give them confidence enough to walk into a casino and seat down with more experienced players. The various Turbo software products are better for this, though, and a lot tougher to beat in my opinion.
Tells are basically nonexsistent.
It's like playing with a blindfold and headphones on. Poker is a people game played with cards and a lot of the 'people part' of the game is missing from on-line poker. But if you're like me and have no other options available, it's better than nothing. Other people have listed various other advantages of on-line poker like no second hand smoke, reduced travel expenses, and other convenience issues. And disadvantages like the potential for cheating.