1) Free book! You're reading all the time, and you have to pay for the things, which is great for the author but if you make as little money as I do and read as fast as I do, it starts cutting into the things I need like food and heat. These books may not be as polished and have fancy covers like the money variety, but you can't beat the price, and
2) You get to discover something new! In the course of beta-reading I've touched on novels I probably would have passed when it came to money. I've read a Sex-in-the-City with gorgons and greek gods, a new england family drama centered around an ancient curse, and the tale of an elven priestess who discovers she actually cares about love and kingdom after all. Discovering new things opens your mind and helps
3) Improve your writing! See other people's styles in their raw form, encounter new literary tactics, clever twists and tricks and everything else under the sun! You can see what didn't work for the author, what didn't work you, and offer them tips to improve. Teaching is the best way to learn, they say, and you get both when you beta-read. While being much more entertained than in a classroom (at least for me).
4) Help someone else. By Beta-reading, you greatly improve the chances that your MS will be beta-read, and the more beta-readers you have, the better your book will be (there's probably an upper limit but let's ignore that) Reaching out to strangers and helping them realize the same dream you have, of getting published and making a bestselling novel, improves everyone's lives, especially yours.
So get out there and read, why don't you! Stop by the beta-buddy page and become one today :D Reach out on Twitter, I'm always happy to give a shoutout. http://nanopals.blogspot.com/p/beta-buddies.html