It's whatever floats your boat, peels your banana, etc. Some people dig rocks, some people dig stars. Some people like finding fossils or ancient artifacts, others like salvaging dishware and portholes.
Cave training is typically, across the board, the toughest training in the recreational realm. I've never gone to commercial dive school or been a Navy diver so I can't attest to the standards required, however I can tell you that the standards required by real agencies giving out cave tickets are at the top of the pile. The mastery of skills required to actually pass a cave course is quite an accomplishment. Coincidentally, that translates quite well over to wreck diving. In that regard, many wreck divers get cave training. While wrecks hold some very specific hazards not found in caves, by and large the skills taught in a cave course will translate very well into the wreck world, whereas not many specific wreck skills translate over to caves. It's not uncommon for wreck guys to take a cave course to learn the skills, even if they never go in a cave again. To some people, rock just doesn't have the same appeal, and to others, a hunk of iron pales in comparison to laying new line where nobody has ever been before.