The reason for them is reliability. Running everything closer to limits really limits reliability.
Aside from that, 1000W is very theoretical. You have some maximum on each rail. If you have 500W on the 12V rail max, and need 600W, it doesn't matter you have another 500W on the other rails. You're SOL.
Another reason is start-up surge. It's less of an issue without mechanical disks, but charging all those capacitors on power-on can really lead to badness.
And droop and ripple are also lower if you're not near the limit. Computers have things like audio cards, WiFi, and other analog parts which perform better with cheap power.
Aside from that, 1000W is very theoretical. You have some maximum on each rail. If you have 500W on the 12V rail max, and need 600W, it doesn't matter you have another 500W on the other rails. You're SOL.
Another reason is start-up surge. It's less of an issue without mechanical disks, but charging all those capacitors on power-on can really lead to badness.
And droop and ripple are also lower if you're not near the limit. Computers have things like audio cards, WiFi, and other analog parts which perform better with cheap power.
I always overspec PSUs.