I probably don't know all of its effects, but one of them is it to enable the "Power saving" setting of each app.What is the benefit of Miui optimalisation enabled for the user?
If it's set to "MIUI battery power saving", this power saving is applied and hibernate the app when the screen is off or the app isn't used any more.
Disabling "MIUI optimisations" overrides these settings: all the apps get "No restriction".
It could be usefull because this "Power saving" setting is bugged with some apps. Some of them get automatically "MIUI battery power saving" back after each reboot despite them were set to "No restriction".
Now, without the ability to disable "MIUI optimisations", we (at least I) have to deal with this forced hibernation.