Most people learn more about how their chosen class/spec works and fits into a group if they play it from the bottom up rather than being thrown into fully-developed character in higher level content. It’s not about the complexity of the game design, but more about learning how to play your role effectively. This is especially useful if someone is brand new to a game or franchise, and therefore has little or no relevant understanding of the game’s quirks.
Essentially, level 1-60 are the tutorial levels where you get abilities one by one and have the opportunity to learn how they affect your gameplay, both solo and in a party.