It doesn't -- at least not in my case.I would be pleasantly surprised if disabling device idle does not have a large negative effect on standby battery drain (eg overnite). But I expect people that are heavy users of their phones and that therefore have also to charge the battery every day - might not notice any difference - and therefore this is a good idea for them.
But, here's the funny thing. I turned on "Power Saving mode" (Battery Saver in stock Android?) with 70% CPU limit, no AOD, and 10% decrease in screen brightness.
I also set WiFi as Metered.
I still got my notifications to come in -- mostly on time.
I can also turn on Data Saver mode if I'm in danger of low battery.
See this as well: https://www.reddit.com/r/GalaxyS21/comments/lqfdhd
Bixby Routines and a widget help me to switch between 1080p/60hz to max res adaptive, etc.
So, I can control how much power is used by the system perfectly.
Still annoyed that, by default, I can't receive notifications without turning off deviceidle via adb.