Most non-nuclear (==conventional) subs run on electric engines with batteries that are recharged by diesel engines. They are much smaller, which can give some stealth advantages, on top of being quieter.

Conventional subs make superior attack subs, and when a sub yet again "sinks" an aircraft carrier in a training exercise its a (often German-built) non-nuclear sub.

Nuclear subs of course excel in staying hidden for months with multiple nuclear missiles. No conventional sub could ever match them at that. But that's not the only thing subs do.