Protect is by far the most used moved in any doubles format and there are a number of reasons for this. The biggest reason if it’s ability to position your self.
Positioning yourself could be chipping one of the opponents into range for another attack to remove them the following turn so they can’t kill the protecting Pokemon. Or it could be pivoting your other Pokemon out so you have fake out pressure or some other kind of support for your protecting Pokemon. Protect is Also useful for avoiding things like fake out. Another use case for it is stalling. Due to the nature of vgc and all the wether, speed control terrain and several other effects it’s also useful at times to double protect to stall something like a turn of tailwind or terrain.
Without protect you also leave yourself increadibly vulnerable to Fakeout and if your opponent realizes you don’t have protect they can easily double targa one of your Pokemon and get easy guaranteed dammage on your team.
