I can run multiple 1080p twitch streams with mpv using streamlink and setting appropriate decoder flags while using chromium to watch even one stream puts a lot of strain on my laptop and gets fan running immediately.

So from my perspective it is very usefull to offload video decoding to gpu and leave cpu cycles for other work. Is it more energy efficient? Never checked that but gpu fan does not really spin any faster and looking at the temperature graphs it does not seem it really strains it.

I tried enabling gpu acceleration for browser (chromium based) and I still don't really know why it is so flaky and unreliable.