Preparations for Sin

Here's a list of all permanently missable items in the game.

• a no encounter weapon, as an equipment drop from Geosgaeno.
• a magic counter buckler, from the hovercraft merchant in the Calm Lands.
• the 0-Slotted Longsword can't be bought after a certain point.
• Master Spheres are limited in quantity.
• the four Al Bhed Primers from 19 till 22 are (technically) missable.

Also, certain non-items are also permanantly missable.

• Having Wantz in Macalania Woods selling 4 slot equipment
• You can't recruit Durren for your Blitz team anymore after a certain point.

For more information, there is a FAQ on GameFAQs giving detailed information for all of these items

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...