Something you'll notice very quickly when you read Pratchett is that, for a lot of his books, he doesn't stay in one place long. Pratchett likes to skip around POVs. The question is, why? (Jenny may chime in on this one, because this bouncing is one of her pet peeves with Pratchett.)
Sometimes, authors use roving POV to create a sense of scale. When you have multiple character POVs, it helps to expand your world. Lots of people in lots of places give the reader a sense of how all the gears fit together and affect each other. Character A sets into motion a chain of events that will trickle through Characters B-D and eventually affect Character E. This can also be a great way to build tension and foreshadow - something George R. R. Martin's POV shifts do often.
Pratchett will use his POV shifts for this sometimes, but often, he's doing it for comedy. Some of his POV shifts are for the express purpose of creating/maintaining a running gag. One character, Cut Me Own Throat Dibbler, pops up in a lot of Discworld books. He's never really a key character, but Pratchett cuts to him time and again. He's going for the chuckle. CMOT Dibbler is a running gag and the POV shift is how he, well, runs the gag.
Also, sometimes the abrupt scene change is enough to keep the reader off balance enough that it increases the comedic impact of whatever follows. If you've ever watched classic Monty Python, you know what I'm talking about.
And now, for something completely different