cheap lessons: you can test out what the better players would do in certain situations. in some ways, any "peer pressure" of a social atmosphere is lessened because no matter what you do, they can get out, so you may as well be as creatively mean as possible. AND you get to try yourself out on their own real, lockup safeties. Finally, you get to feel the adrenaline and pressure of "if i miss, they're not only running out, they're running racks" that you just don't feel in lesser tourneys.
variety: we've got some great players with whom i would never be able to play outside of the open tourneys... gambling with them would be just giving my money away, and any game i tried to make for more even odds wouldn't have the same desperate feeling of near- hopeless gotta-get-a-game pressure.
hope: better players play prettier. their stances, patterns, strokes, rhythms, everything gives you something to aspire to. you get to see how the game could/ should be played, with the dynamic exercise of playing in the same match that you don't ever capture in just watching live or on tv.