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.