It shouldn't show you as online in discord/slack, but it should be downloading messages/etc so that when you do come online you don't have to wait for it to sync with all your cloud services.

Also - consider those cloud services might not necessarily be available when you come online - maybe you open your laptop on a train in an underground tunnel or something.

Macs do a good job at this. They have "high efficiency" CPU cores which are still very fast (like, very fast*) but draw about half as much power as the regular cores. Software is also able to schedule background tasks based on various things like power level, network connectivity, how often the user actually launches your app on this device (maybe you have an app installed on all your devices but only actually use it on your phone...).

Background tasks like checking emails, backing up your computer, installing security patches, etc will all run while your Mac is sleeping.

Anti-theft features run even fully powered off. So unplug the battery, and never plug it back in, if you're going to steal anything with an Apple logo... the fact you can never turn it on does hurt the resale value, but that's better than going to jail. It'll phone home as soon as you boot it up too, and even after a full factory reset is still probably tied to the actual owner. You'll need the owner or Apple to deregister it - and Apple is likely to call the cops unless you've got a good story.

(* to give you an idea how fast the "Efficiency Cores" are on a Mac — in Game Mode the "Performance" cores are powered down, because the efficiency ones are more than fast enough and generate less heat - which allows the GPU to be pushed to the limit of the cooling system. The "efficiency" mostly comes from reducing features like speculative execution... though they do also run at a lower clock speed - as in ~3Ghz instead of ~4Ghz)