Preamp function.

Hello everyone. I have always wondered what a preamplifier does. I am currently investigating valve amplification for the first time. one of the options i am also considering is a valve preamplifier to add to a pass labs power amp. i am also considering all valve. Transistor or valve, what does the preamp actually do, apart from switching and volume control. As an experiment i added a musical fidelity a3cr that is from my second sytem, to my pass xa30.5. the results were very positive and an improvement over cd player direct. i am aware the musical fidelity a3cr is not in the league of the pass, but it certainly improved the sound. Most noticeably was a quieter back ground and better musical presentation. From this i gather that a preamplifer does add something to the sound. i had always believed it purely carries the signal unaltered. If anyone here with technical kwowledge mind throwing some light on the subject, i would appreciate it( others might find it interesting too). i have read reviews where writers describe the sound of the preamp. i had believed they were actually describing the sound of the player and amplifier and the ability of the pre to carry the signal unaltered? Thank you.