The same way some pople use their integrated amps to bi-amp, or use it as a pre-amp.
The seperate power amp is of higher quality than the one built into the receiver. Imagine you have a £1000 receiver, that's $500 on the processor side and $500 on the power amp side, many people like their connections and such on the current receiver, so they just use it as a processor then use an external power-amp for the sound upgrade they're looking for.
hi im fairly new to the home cinema scene and am just curious to know why some people have a power amp as well as a reciever?
I would say that it's more than likely because they'll want to run the front 2 speakers in stereo as well so powering them from the power amp will give them that option (if it's a stereo power amp that they have etc...)
The reality is they probably don't need to add the power amp if they have the right AV receiver. A number of AV receivers allow you to assign the internal amps for stereo duties - so you can bi-amp the Yamaha RX-V667 to drive 90w to the high and low frequencies of your speakers. Likewise, the Onkyo TXRS608 which drives 135w to each channel, thereby providing additional oomph through bi-amping there. No need to buy a separate power amp accordingly.
The main reason is the bang for buck. A separate processor and a multi channel Power amp (in most cases) is deemed to be superior SQ when compared to a standalone receiver (due to many reasons including sharing of the same Power supply for both processors and power amp modules in the receiver which might not cater to the demands of Power requirements in demanding situations! )
A mid priced receiver with pre-outs is generally cheaper than more or less on paper equally specced to stand alone processor (processor is meant to be more audiophile and hence there is an audiophile markup but also people who buy processors believe that the components used in them are better quality than the receiver).
Hence people have a receiver and a Power amp - Bang for buck!