They will be the third layer of bitcoin in a far away future (remember, we are early). Communities, cities, regions will all have their mints where residents will be able to transfer theirs bitcoin (supposedly from lighting, layer 2) and use them in the community with very very low fees and instant payments. That's because lightning would not be able to support all that much people directly and it might be possible/feasible only to use it to "charge" your mint wallet. Of course if you move from area to area it will be possible to transfer to another mint using lightning and also from mint to mint directly due to mint to mint agreements.