The theory behind it is that by keeping the pre & power sections apart, both are able to attain higher quality.
The pre is needed to do source selection (CD, Tuner, Aux, etc, etc) and volume control. The power is the bit that actually does the amplifying.
You can't use one without the other. Only exceptions being adding power amps to integrated amps (as I have), which allows bi-amping of speakers (Integrated does the tweeters and the power does the woofers). But not all integrated amps have the facility to add a power amp, and you need speakers with twin sets of terminals. The other exception is you can use a pre-amp with active speakers as they have their own power amplification built in.