My guess based on the timing is that it is primarily for the Steam Frame. Steam is not just a game launcher but the entire desktop environment for the Steam Machine, the Steam Deck, and anything else like Bazzite that uses it as its primary UI as well. But in those cases, while at scale every second saved is nice, for the most part a second here or there doesn't really amount to that much.

But for the Steam Frame, which is probably booting while it is strapped to your face and completely obscuring your vision, every second is felt much more intensely. Anything that can be done to get to first interaction sooner has a much, much larger effect on the feel of the experience.

The speed would always have been nice to have, but it moves beyond "nice to have" for the Steam Frame case into having a significant effect on how it feels to use and the out-of-box experience. Yes, you'll use sleep most of the time, but when you do a full boot or reboot, you feel the seconds.