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

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.  

duh

Gain structure, and the Janus faces of impedence/resistance. Also known as, the rwo most oft misunderstood concepts in consumer audio! See the excellent Stereophile article linked earlier in the comments. -cheers

Once you hook up your input devices to a dsp or pre amp you will hear why.The pre amp section in most amps cant compare to a dedicated pre amp like a rotel RDP-980.That is what i run into my HK-5000 and the tone is superb.On the other hand some amps have limited inputs so a good pre amp is the answer.Colin

I wish i could post a pic of the inside of the rotel, totally amazing..i run a retro repair center and seen loads, most new type pre amps have 3-5 8 pin audio ic's, the rotel and many older units have a full pcb packed with semis and ic's.