Well, we don't really actually know all that much about the actual composition of comets. They're not quite asteroids-aka big balls of rock or metal. They're sometimes described as big space snowballs, which while being a gross oversimplification is kinda close to the mark. But besides that, we didn't even know if we could even land on a comet until yesterday-there were worries that the probe would simply sink into the surface, or any other number of catastrophic events. Now that Philae's up and running, we're going to get so much stuff about what makes these things work.
Like Kay said, it was also practice for other things. The probe carried an experimental harpoon landing system. Basically you fire at the object and reel yourself in with it. I don't think it actually worked (I believe it failed to fire?) but it's an idea that's been suggested to use when landing on smaller bodies. Remember, shit like asteroids or comets have negligible gravity, so even if we can get into an orbit around one, that doesn't necessarily mean that landing is going to be easy (or even possible) with the aid of what little gravity there is. This is a system designed to land and attach probes (and maybe eventually spacecraft) in situations where you can't trust the object you're landing on to keep you stuck to itself.