Technically, any head or combo has both a preamp and a poweramp. They're just words to describe particular types of electronic circuits.
The preamp's job is to amplify your guitar's signal to a level strong enough to drive an output circuit. Generally this means amplifying the signal's voltage. They typically also have tone stacks to control bass/mid/treble, and will most often have some type of overdrive circuit to create overdrive/distortion (depending on the amp).
The poweramp's job is to amplify the signal from the preamp enough for it to drive a speaker. Because the speaker is a low-impedance load the poweramp needs to amplify current, as there's going to be a fair amount of it flowing in the circuit. Some of them have tone stacks (might be labelled "presence" or "brilliance").
In many cases a head/combo's effects loop is just a couple of connectors in between the preamp and poweramp. In some they're not, e.g., the Custom Audio Amplifiers' OD100 the effects loop has a tube driver stage.
Now, why do people use these instead of a head or a combo? I would conjecture that it is for either of two reasons:
I'm not going to argue that a combo or head sounds better or worse than having a separate preamp and poweramp. It might sound different, but if it sounds good it is good. Some players, myself included, get their favourite sounds from having a separate preamp and poweramp.
Flexibility can also be a reason for choosing to have them separate. Some of the rigs used on tour, and quite a few also for studio use, are really complex. They might have a number of pedals in front of the amp, as well as some between the preamp & poweramp (aka "the effects loop"), and switching circuitry to go from one sound to a completely different one by touching one button. They allow you to change just one piece of the system should you wish to do so (e.g. your preamp manufacturer came out with a new model that has more sound options). Also, if everything is in a rack it's easy to move from venue to venue.
If some of this was confusing, let me know and I'll do my best to clarify.