Like what? Well something
Most people don't play the game for specific purpose or goal, they just play to spend their time relaxing or spacing out. At that point, it does not really matter what you hunt for, just that there is something to gain
People play for many kind of reason and some people are what we called objective-lead. This means, that for them to enjoy a game, there has to be a clear objective or goal. It does not matter what that objective or goal, just there is something specific to go after. This is again something which AP provide. It provides a goal to hunt.
So it does not really matter that mythic raiders get a huge reward, just that there is still a reward to be gotten by the casual playerbase. Every part of the WoW community needs some kind of constant reward for hard work, and since we have a game, that is based mostly on fighting, then that reward have to have an impact on that fighting
That is where power comes in. You ask why casual players need legendaries, AP and TF? Well, its because it is BLizzards answer to the need for increasing power. I personally think that they could have done it other ways, like only having AP work outside of raiding and WF/TF and legendaries only dropping in raids, but i don't control the dev team and decide on this