In short, difficulty is what forces you to actually learn a game, and the absence of it is the absence of that motivation.
For example, if you are playing an MMO and a boss puts down red circles of fire, with the mechanic being to step out of them, that mechanic only really works if you must step out. If you can beat the boss while still standing in the fire, or getting out slowly, because the difficulty is that low, you aren’t motivated to learn the mechanic. In a vacuum that might not be a huge issue, but if the overall design of the game hinges on players learning and appreciating the mechanics, the difficulty being too low ruins that entire design, regardless of how good the actual mechanic is. Responsive controls so getting out of the fire feels skillful, interesting abilities to assist in moving out, gearing up correctly so you buy yourself more response time, etc, none of that will matter or feel important if the game doesn’t punish you enough for ignore said fire.I think this is partially correct, and partially incorrect. It's certainly true that a lot of mechanics at LFR level are totally ignored, and thus the level of play is very low. But a similar phenomenon also happens at the very highest level of difficulty.
For example, in the Mythic M.O.T.H.E.R fight this tier, there are three types of adds, each with a special mechanic. I have no idea what those mechanics are, because we send the entire group through the field at once, survive with major defensive cooldowns, and blow up all the adds instantly with a full raid's AoE.
Similarly, on Mythic Taloc, there's some sort of "stack on the tank" mechanic. We have a paladin cheese it with bubble and Spellwarding. On Heroic Zul, we have a tank suicide off the edge in Phase 2 (and then battle-res) instead of dealing with the blood on the floor. Heroic G'huun is one long exercise in using movement abilities to avoid having to throw the orb and clear paths.
This happens in other games too. There's a host of high end fights in FFXIV where you can ignore specific phases if your DPS is high enough ("skip soar"), and the community goes to great lengths to increase DPS so that they don't have to learn those phases. (This may be the NA/EU community only. My understanding is that the Japanese community actually creates and learns strategies for those phases.)
In some respects, high end gameplay is all about figuring out which fight details can be safely ignored. If you really want to learn all the small details in a fight, the sweet spot is probably around late Normal or early Heroic mode, when your players aren't good enough to brute force mechanics.