Why do people buy preamps if they can directly connect amp to stereo source/phone/PC/tv?


The purpose of preamp as I know it is to boost the signal to make it stronger. But I have never used preamps and my current system is already too loud (and sounds great) at moderate volume without preamp. Why do I see many people buying preamps even though they have powerful power amps?
mrbananapeel
In addition to everything already said a Preamp allows control of bass, treble, channel etc. which most power amps cannot do 
A good preamp can maximise the source’s potentials by revealing the details in greater resolution (if the speakers have enough accuracy and transparency).  In some ways it can be described as the soul of a system.