I thimk the idea behind it is to make raids beatable, but not too beatable. The alternatives are either bosses that do a lot of damage all the time or bosses that do their standard damage amount all the time. The lower damage option wouldn’t work for the developers, since it would mean any team that can survive the standard damage would be able to keep attacking the boss indefinitely. Conversely, extremely high consistent damage may frustrate the player base. I personally would support bosses that always hit very hard and your team just needed to figure out how to handle it, but my guess is the enrage mechanic was more of a compromise between “too easy” (from developers) and “too hard” (from players).