I heard of leaving the car on but not started from 10 min. to 8 hrs. won't that drain your battery? also what if I disconnect positive cable? I just bought a 320 clk 1999. please help.

 

I have two questions.
Why would you leave the ignition on without starting the car? Especially for an extended period of time.
Why would you want to disconnect the battery? Are you placing the car in long term storage?