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

I am not an engineer and speak from personal experience as well as from consulting several audio engineer friends.
You need a preamp in most cases, primarily because it provides extra drive from the source to the power amps. Even if you have enough gain, the preamp does impedance matching so it creates synergy between the source and the power amps.   The volume control is also critical and in most cases, using a DAC with digital volume creates loss because of losing bits at lower volumes. The preamp is also useful for switching between sources if you have multiple. I had my share of experiments with different passive preamps and was always disappointed, because of hearing different anomalies (lack of dynamics, body, extreme brightness or unbalanced sound etc) so always returned to the active preamp, which, despite its flaws, better than going passive.

Zoltan

I've used high end DACs with built in volume controls and always found better performance using a dedicated preamp.

 I agree  with others here.... the question is not specific.

I've been dabbling with stereo equipment for almost 50 years and the majority of the time I've had separates. The versatility of a preamp is key for me ....like when I had my McIntosh C26 ...the stereo reverse or R&L to left or what ever gives me Jimmy Pages' guitar reversed on or off to only on or both speakers. Now I have a C46 that gives me a fine tuning feature with a built in equalizer and balanced connections for clarity. Another reason is that you can try different power amps for more versatility. Couldn't live without the preamp in my world.

an integrated is a preamp + a power amp

No matter what you do you're going to need the functionality of a preamp. Which means that the sound of your system will be affected by the preamp in an integrated, in a DAC with preamp, streamer, or whatever you use. I have used my headphone amp as a preamp because it has two RCA outputs. It sounded terrible.

I think a preamp has the greatest affect on a system's sound. I have demoed a passive preamp, which does not use power. It simply chooses which input to feed to the amp and has a volume control. Theoretically, it is the proverbial straightwire that does not affect the sound. I didn't like it. The sound of my system became flat and the soundstage collapsed. When I went back to my ARC preamp, the soundstage inflated and instruments had bloom again. One might say that I preferred sound distortion. So be it. I much preferred my ARC to the passive preamp.

My point is that the preamp is an important part of your audio system, and you're going to have a preamp no matter what you do. I always think you should use your ears to evaluate a system component. Compare an integrated to separates in the same price range. See which one you like best. In other words, just because the preamp is built into an integrated doesn't mean you no longer have to think about how it affects the sound of your system.