Sampling is simply recording audio. What is meant by sampling in this context is that you nick a bit of somebody else's recording - this is how hiphop evolved; early hiphop DJs would have two copies of the same song, and then mix between them, so as to allow, for instance, a four bar break to carry on ad infinitum, which MCs would then toast over. When samplers came along, they recorded the grooves/breakdowns they liked into their new digital machines and loop them in there. This gave rise to a whole host of new opportunities: layering of different breaks, effects, and most importantly chopping up and rearranging a sample. I wish I could point you to some early hiphop tracks where these techniques were obvious, but I'm not really knowledgable when it comes to old school hiphop - although I do remember that Public Enemy's Fight The Power (and a million and one other hiphop, jungle, d n b, and 2-step tracks) blatantly samples the breakdown in James Brown's Funky Drummer.
Why should you sample instead of playing you own hooks? Well, you shouldn't, necesarily. But take a listen, for instance, to a sax line on a John Coltrane record, and compare it to something you play yourself on a Triton with a multisampled sax sound... not quite the same is it? What I often do is nick a melody line off a sample CD (to avoid copyright issues), and chop it into small pieces, which I then rearrange and mix and match with other snippets of melody lines. I do the same thing with drums - there is something about the sound and feel of a break played by a human being that can't be recreated with a bunch of one shot samples and a keyboard.
If you have any further questions, feel free to ask - I can most likely help you with anything that doesn't relate to the MPC

(I don't own one... yet...)
-Paws