Wow. People answered. With... like... useful stuff. Too late, I guess, but my answer would have been "If you have to ask, then you shouldn't".
That isn't being mean, either. Too many people brick their systems because they don't know what they're doing, or they let their children (who don't know what they're doing) play with it. Installing b9s gives you the ability to do pretty much anything with your system software, as well as run any code. This includes deleting a system file that's required for your system to work. This is just one of many examples. Some people really shouldn't do it.
And don't let anybody fool you. Themes are fun to play with, but NOBODY hacks a 3DS just because they want themes. Stealing games, playing emulators (my favorite reason), and the ability to cheat, probably covers the MAIN reason(s) for 99%.