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That's called staggered spin-up. One does that for sets of HDDs, too.

Anyway, what is it good for? To not overload the PSU for one, because spin-up needs more energy, until it has reached it's intended range of RPM. (for fans & HDDs)

Another reason to spin-up high initially, to fall down to slower when up, is to overcome friction in the bearings, to get stuck things unstuck, and shake dust off. (for fans)

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Tell that to my switches and PC servers. They all spin up their fans in one go as soon as power good is received and the initial BIOS thingie has stubbed out its fag on the floor.

The PSUs are painting their nails when the fans go off initially. The GPUs, RAID controllers and co are still waking up.

A further counter example is an Equallogic PS6500. That has 48 3.5" spindles in it and three PSUs plus a few fans and two controllers. The whole thing boots in a couple of minutes.

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