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.