hi instead of closing them and letting people go to them and wanting to reply and find out its closed just delete the thread it would be so much nicer
Closing the thread allows people to know what and why it was closed, usually.
If it was deleted, that wouldn't be possible and then someone would just post (as opposed to PM) why it was deleted, and then that thread gets locked.
There are some threads that just get deleted immediately, mostly advertising though.
Pretty much what Remilia said. Visible moderation is good, and deleting all bad threads would just cause confusion.