Take it from someone who has a vested interest, other than just fashion, in keeping their leather jacket in good condition.
Cows are still alive. Living skin continuously produces oils and is self-conditioning. Once the leather is "dead," that does not happen.
Water, among other things, dries leather out.
Whenever I get caught in the rain and my jacket gets wet, it has to be reconditioned. Repeated whetting will dry leather prematurely and it will crack. It also loses abrasion and tear resistance if it is not reconditioned after becoming wet.
However, leather can get wet and not suffer any permanent damage if it is allowed to dry slowly and then oiled or conditioned. One of the worse things you can do to leather is to get it wet and then apply any kind of heat to dry it.