If you are really curious I can give you a few use cases I've seen that I think are valid.

1. There is a gap in housing between when someone moves out of their old place and when they can move into their new one. So they need to store all their stuff for a while.

2. Someone has a nice hardwood tree taken down in their yard and they want to keep the wood, but they don't want their garage /workshop to be full of drying wood.

3. Bulky outdoor sports equipment. Storing your kayak, etc. in your urban apartment may not be an efficient use of space.

4. If you are in business for yourself, you may need a small amount of warehouse space. The easiest way to get that in the US? Self storage.