Sierra1

Question: Why did the old T-800 prevent his younger self from killing three punks and stealing their clothes? I mean, these punks were meaningless to it, so why did it save them?

Answer: The T-800 "Pops" was working with Sarah Connor, and they were trying to eliminate any Terminator sent back in time. The punks were incidental and the aim was not necessarily to save them, but Sarah probably also wanted to minimise the collateral deaths as much as possible by destroying the machines as soon as they emerged.

Sierra1