When your laptop gets to critically low battery it has 2 options: Power Plan Settings - Change
It can hibernate and save whatever you were doing on screen or
Shutdown, not saving any of what you were doing.
The one thing it can't keep doing is stay switched on there isn't enough power for that.
You can tell your laptop to warn you when the battery gets down to a low percentage (maybe 10%) which would give you enough time to either finish what you were doing etc. or plug in the mains lead.
If you were to somehow disable auto hibernation then whenever your PC reached critical battery it would just keep running until there was literally no power left and just stop possibly causing major hardware damage.