Because up until 2.0 launch, there will be a storyline occurring throughout Eorzea that will not be replayable. Those currently playing will be able to play through the storyline leading up to the what I would assume to be the coming of the seventh umbral era. Chances are we'll find some way to divert it, and the world will only be heavily mutilated in the process instead of completely destroyed. They said that come 2.0, FFXIV will shift to a new storyline.
With the current storyline, the last big battle being with one (or maybe both) or the Garlean Legatus says enough. I think I have what's going to happen figured out, personally. At least somewhat.
But, to sum up, essentially, we will get to play through the events that will bring about the new Eorzea. We'll have experienced something none of the new players will ever have a chance to - and we'll be developing our characters all along. If you want to come back to a new game, that's great. A lot of players will remain behind to play through the changes, though.