Dream Zanarkand Destroyed - Why?

Hey, this is my first post, just got really into FFX again, I'm just approaching the final battle but I was wondering if someone could clear up why the Dream Zanarkand got destroyed at the start of the game and why Auron pulled Tidus into Sin? I understand the whole back-story about Bevelle vs. Zanarkand and the whole Yu-Yevon/Lady Yunalesca thing. And I get the whole Jecht getting pulled to the mainland by Sin cos it was docked near DZ and he got too close. But I'm a bit confused how a city based on a memory got physically destroyed by Sin when Sin was built to protect it.

Jecht did it to prove a point, that his power was so destructive, that he had to be stopped. For now, his power was somewhat under control, but when he lost sight of who he was, he would be come Sin fully, and he would not be himself and continue to destroy mindlessly for the glory of Yevon. While he still destroys currently, he is somewhat inhibited in ability, and able to think for himself, not that he is allowed to act on it at all times.

Auron took Tidus so that he would be able to travel to Spira, since Tidus knew how his father thought, he would be able to stop him. Jecht had also asked Auron to look after him, and make sure that he did not grow up to be a crybaby (nice job, Auron, quality work that turned out).

I apologize to the FFX insane-os who hang onto every detail and intricacy of the plot, if I've oversimplified it.

There's no need to go into detail about the finer points of DZ/Sin/Aeon connections, but to put it simply, Sin wasn't built to protect DZ, Sin is a weapon. An almost invincible weapon that has remained in use for 1000 years. Yevon acted independently from the Fayth to build Sin, but he still needed their power, which explains (partially) that when Sin is destroyed, the Aeons are used in order to let the Fayth rest.

Thanx for that, don't worry about the hardcore fans, thats explained it pretty well. It's just such a complex plot, I haven't even picked the game up for about 4 months so I'm clutching at straws to understand some of the latter aspects of the plot.