wht does a preamp do?

I had been running my wadia cd player direcrlty into my amplifiers. I just added a preamp and am very surprized at the differnce. The bass tightness and frequenct extension are very different. I am also hearing a 3 dimensionality that i never heard before. clearly a preamp is not just a volume control. Wht else is it doing to change the soundstaging so dramatically?

Rum