Any mild acid will do wonders for tenderizing meats-- the acid loosens the collagen bonds, which are further relaxed by mild heat (cooking).I agree about tenderising.
I most often use just olive oil and white wine vinegar as a marinade. I find that to be particularly effective with pork.
One reason why slow cookers tenderize meats: the relatively mild temperatures break the connective tissue (collagen) down.
High heat does other things; lovely carbonization; carmalization of sugars and starches. That oh so taste browning effect we love so much.
But in a medium rare steak, the heat in the middle never reaches super-high values, so marinades can do that for you.
Of course, a well done "steak" kinda supersedes all that prep work, and makes me ask, "why bother"...
... may as well have a well done hamburger instead.