Major Final Fantasy X spoilers below.
So at the start of the game, Sin pretty much obliterates Dream Zanarkand, the "present" Zanarkand Tidus is from. In the middle of the game that make sense; Sin destroys stuff. Of course he wiped out Zanarkand.
But late in the game we find out Sin was made by Yu Yevon who also created Dream Zanarkand, in fact he created Sin explicitly to protect Dream Zanarkand. So why did Sin destroy it at the start of the game?
I've been digging through the Final Fantasy Wiki and haven't found a bit of explanation for this, and I don't recall a reason from when I played and complete the game, though that was years ago now.
Best Answer
I may be wrong but since Sin is Jecht, wasn't it to get Tidus 1000 year in the future. When we get swallowed Auron says something to Sin like : Are you sure? It is decided then. And then they get swallowed. So yeah I would bet to bring Tidus into the real world a 1000 year ahead of his time so that Tidus may kill Sin (Jecht). And at the end of the game when we're about to face Sin, Tidus is on the ship and he sees Sin screams and Tidus is like...You want this to end don't you?
So yeah I think that's why he "destroyed" dream Zanarkand, just to get his son to kill him. He probably thought no one else could do it and maybe he thought that only Tidus could end the cycle of the Death of Spira...