I've played this game for about a year now, and basically, I can't tell what the point of powerful gear really is in the game (and by "powerful" I mean the power level of gear). Note, I'm not particularly active in Strikes, Crucible or Raids.
From what I can gather, some creatures take reduced damage from either the difference between your level and them, or your power level and them. But once you're at sufficient level (or power level), everything still takes a "couple of hits" to kill off. Power level doesn't seem to make any difference at all. The basic mobs that you encountered at level 5 take just as many hits to take down at level 5 as they did at level 20 (and I'm only just now starting to play the Osiris, Warmind, and Forsaken). Things still take 1-2 headshots, and 3-4 body shots to go down with my level 325 gear as they did with my level 100 gear.
Google searches about "what does power level do" only result in "how to maximize your power level".
Is it really just a gating mechanic to prevent you from doing content unless you've ground long enough, and not just because you do less damage and take more damage?