I guess you can have smaller motors to drive you around on the bottom, but the vast majority of the travel is controlled via ballast/buoyancy.
To me, planes and cars have much more obvious gains from lighter materials. Not so much for the sub.
It's always a cost tradeoff - I think Challenger Deep (Cameron's former sub) was steel construction which required a large volume of syntactic foam that can resist high pressures to counteract the weight of the steel. That makes for a much larger vessel.
Titan's submersible for exploring the Mariana Trench I believe is largely titanium so much lighter than steel, but at a hugely increased cost.
Not having to bond expensive foam to it to make it buoyant. I also think it may have something to do with the shape of the hull in this case, but I have not seen any commentary on that yet.