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

Choice and sound quality. You don’t "need" a preamp.

A good example of having separates is an electrostatic speaker. You love the sound of tubes, but the tube amps tend to be poor performers (queue the chorus of A’goners who will "not always.." this post) so you mix a tube pre wiht solid state amp.

Also, simply power. Having separates allows you to up/downgrade your power size and try different amps (D, A, A/B) without changing the pre.

In a sense, it's similar to having a separate DAC from your preamp.  Having these separate lets you change your DAC while keeping everything else. 

@erik_squires  I agree with your statement.  I run SoundLab Majestic 745 speakers.  For power I use the Hegel H30 SS mono blocks.  I had the matching Hegel SS preamp for a while but found a significant sound improvement (IMHO) when I introduced the BAT Rex 3 Line Stage tube preamp.  In my application, it's the best of both worlds.  Cheers.  

I’m ready to admit my favorite tube preamps are a "coloration" (i.e. a pleasing distortion) added to the sound. But I really, truly prefer the resulting sound with them in the chain. If you want something that is a straight, transparent pass-through (i.e. "wire with gain"), then solid state or passive is the better way to go and I think there are many fine preamps which can get you there.

Back in the day of "just" analog sources, a preamp with some extra gain could be quite useful. Now we have digital sources that can push out 4 - 10 Volts from their XLR outputs, and your preamp is simply acting as a fancy attenuator (to be sure - attenuator quality matters!). You might as well look at something like a nice Khozmo passive, if you’re doing digital only (being careful to match impedance between source and amp, the goal being a 1:10 ratio on each side).

If you’re doing BOTH analog and digital sources, then you might have an awkward dance of optimizing for both of those with one preamp. Though these days, there are plenty of options for getting the analog source levels up on par with digital - e.g. using a SUT, or a high gain phono like Pass Labs or ARC Reference.

I can't imagine life without preamps.  I own several.  Actually most of us who play vinyl records have two, because the phonostage is a preamp, or a pre-preamp if you want to think of it that way.  Then there is the line stage, which usually has all the controls on it, volume, input selection, outputs for amps and subs and so on.  Think of it as your control center.  An integrated amp has all of these things "integrated" into one box, but all the parts are there.  So for most of us, having a preamp, or two is absolutely the way of it.  I will leave those you want to work around it to their own devices.  For the majority of us, having preamps is sine qua non, that is latin, meaning: without which nothing.

Active preamps have always provided the soul for my systems, performers become  real live flesh and blood, totally indispensable for me.