There was a solid analysis of these facilities done by Modern MBA. https://modern-mba.com/case/self-storage

tl;dw - As far as businesses go, they require low capital to construct, have low operational costs, and also operate as a way to hold real estate/land while also generating revenue.

The people that use them are usually dealing with some life event such as moving, a death in the family, divorce, and just want some solution to house possessions.