This skips all the content before the previous expansion to give players the most modern experience from a quest design POV (and to make sure the players are connected with the current storylines). It also ensures new players have a shorter leveling ramp—Blizzard says it will be as much as 70% shorter than it is currently to get from level one to the cap.
Keep raising the level cap with no changes to the leveling experience up to that point. This leads to ever-lengthening leveling ramps that make it extremely difficult for new players to get to current content. That's because each expansion adds another several dozen hours to how long it takes to get from the start to being able to play with friends in the endgame. (Final Fantasy XIV has this problem to some degree, even though it's nowhere near as old as WoW.)