I used to have everything different, going for the "best" preamp, power amps, tuner, CDP, etc. Only SQ mattered, "matching" was irrelevant. A "system" from one company didn't figure in my thinking — it was "my" system, unique and eclectic.
I'm currently I'm using a Tandberg pre + power. They were designed, not only to look similar, but to look like one. Both are beautiful, the spare but elegant Scandanavian look, but together they blend into one, the contours, controls, graphics etc flow into each other, the two units together don't look like separates, they look like the most powerful and gorgeous integrated amp ever. I've used each separately with other components, they sound fantastic — but not better, and the unity of aesthetics is so seductive.
I suppose there are electronic benefits too, matching impedances et al, but I've no knowledge in those areas.