Settings & keep-time · Quiet Hours, pausing and schedules
Should I use the Always on mode in Quiet Hours?
Short answer: Use Always on only if you don't mind automatic work running at any time, even while you work. Smart mode does the same work but waits for a quiet moment, so it suits most people better.
Only if you're happy for automatic work to run at any moment, even while you're busy. Always on never holds automatic work back, which is why the app says to use it with care.
For most people Smart is the better choice: it gets the same work done but waits for a moment when you aren't typing, aren't in a call and aren't low on battery. Always on can make sense on a desktop Mac that's often left unattended.