Dumb question......why do you need a preamp?

I have used active, passive and just volume controls on components. The best sound without question, for me, has always come from a active preamp. The most disappointing for me has been the performance I have gotten from several very high quality passives. 

I think it is pretty obvious why an integrated amp, in most cases, is not going to sound as good as separates. I know of several people that drive their tube integrated amps with a separate tube preamp. In my experience the preamp section rarely matches the quality of the amp section or vice versa. The only reasons to go with an integrated would be size, cost or simplicity.