If it’s bad to close off air vents in specific rooms, why do they ale vents to be closed off?

Like what is the point of the ability to close of air vents if everything says you should not do it because of pressure buildup, etc?

And if it’s not bad, can I safely partially or fully close two vents in two unused rooms at the corner of my house to allow more air flow into the used rooms and ideally less heat usage bc more focus is spent on warming up the two used rooms vs the two unused rooms?