Some people have been attacking the existence of multiple raids. Let me say straight out: multiple raids are awesome. The real reason behind multiple raids is one that is inherent to the nature of gaming and gamers, that is, that there are multiple skill levels.
In particular, some would argue that the skill disparity has increased, as throughout WoW years good players have gotten better and better, and new players are just as clueless as ever. Population wise, and this is significant too, a large bulk of the WoW populaton has increased in skill level, and the general graph tends to move more and more towards high skill level, as long as these vetarans keep playing and improving.
But regardless of the population situation, there will also always be the new guy to be adressed.
Well, here comes multiple difficulties: they allow players in many different skill levels to experience actual raiding, that is, being challenged and working as a group to overcome. More importantly, they allow players in many skill levels to improve their skills and they create a clear path and measure. They build a large and well tiered stairway of skill that players can climb as they become better pve players. With a single difficult you could also have a in-tier difficulty, but it would either start too high in difficulty, or stop too low, or be too drastic, with huge skill jumps between bosses. That is, any solution would hurt some section of the playerbase, and any solution would hurt the skill stairway itself.
Now, let me say again: multiple raids are awesome. But, as many other good things, they need to be well applied. World exploration, solo playing and dungeoning are all important aspects of the game. They should all be integrated with raids in a system that estimulates and rewards both character progression and player skill improvement. Players should be encouraged. as they hit max level, to do harder and harder dungeons, practice their class a bit, before setting foot in a raid, which should, let's agree here, be a place of epic and challenge. There should be a smooth difficult transition between the last dungeon boss and the first raid boss. But raids should feel hard and challenging from the very start.
It's needless to say ofc that lfr is an atrocity that shouldn't exist at all. It hurts the game in many ways, it's not really a raiding experience and it's totally off the skill stairway. The first raid difficulty should be a challenge to a group of new players that honed their game running dungeons. It shouldn't be an insult to the player intelligence, or a downgrading of the enviroment and bosses.
Players in a higher skill level shouldn't be forced or even estimulated to run lower difficulties by any means (such as valor farming etc). It's a boring experience that speeds burn up, let they only do it if they want to.
Raids themselves should be tiered throughout the expansion, it shouldn't happen that a raid instantly becomes irrelevant as soon as a new patch hits. You just got to max level in the mid/end of the expansion? You'll first have to beat the first raids before setting foot in the last. Forcing everyone to just do the freaking current raid is a way to quickly burn out people and waste your raid creations.
You should always deliver content to the veteran playerbase, which is large, so even the first raid should have tough mythic encounters. So multiple raid difficulties and multiple raids form the stairway of pve skill.