Learn Polish: Oni powyłączali wczoraj wszystkie monitory. - They turned off all the monitors yesterday.

Powyłączali uses the distributive prefix po-, which emphasizes that they turned off the monitors one by one, or that there were many monitors involved. Wyłączyli just states the simple fact that the monitors were turned off, without that sequential flavor.

To make this distributive verb, why do we add po- to the imperfective wyłączać rather than the perfective wyłączyć?

That is a standard rule for distributive verbs! When creating a verb that means doing something to multiple objects one by one, Polish attaches the po- prefix to the imperfective stem. So the imperfective wyłączać becomes the new perfective verb powyłączać.

Does powyłączać always require a plural object like monitory?

Yes. Because the po- prefix in this context specifically means doing an action to multiple things distributively, it requires a plural object (or sometimes multiple subjects). You cannot use it to say you turned off a single monitor.

Since monitor is a masculine noun, why is the adjective wszystkie instead of wszyscy?