Is this ever used for anything? I started into modular about 5-6 months back, have filled up the better part of a 9U 104 HP rack, and have yet to purchase or encounter anything that utilizes the 5v rail in any way. Am I missing something? Can someone explain its purpose or why many PSUs seem to dedicate even more power to the 5v rail than the seemingly much more important -12v rail? A bit confused about this obviously.

there was a period when a lot of digital modules took power from the 5v, but the trend now is to convert +12v to whatever is needed by the mcu (usually 5v or 3.3v)

these modules were often quite power hungry too

most modules use comparatively little -12v (except video, argh)

if your psu has a lot of spare 5v and you have some space in the rack you could consider a usb power module (there are a few available) and charge your phone, power a led lamp etc

Utility modules are the inexpensive, dull polish that makes the expensive, shiny modules actually shine!!!

On some actual module you can choose 5v converted from 12v or using 5v from power bus, with a jumper at the back of the module.
If you use 5v from power bus it lowers 12v consumption.
And some like Klavis Quadigy doesn't run without 5v.

Interesting @klodifokan I’ll have to check and see if any of my modules have that option. I’ve got some power to spare for the moment but if I can unburden the 12v rail at all that’ll be helpful long term.

Thanks for the info @JimHowell1970