It seems you're imagining them full of someone's hoarding overflow, obviously egged on by reality shows. Honestly, pretty judgmental and uncharitable. I'm pretty sure the more common use case is related to moving, life transitions, and housing insecurity.

Where do you put your stuff when you can only afford to rent a 2br apartment, between 4 adults? A storage unit.

Where do you put your stuff in the summer as a college student? A storage unit.

Where do you put your stuff when an entire city moves out on 8/31 and moves in on 9/1? A storage unit.

Where do you put your aging relative's belongings when they are suddenly unable to care for themselves and need to move into assisted living? A storage unit. What, you don't seriously believe their kids have a house, do you? In this economy?! Who can afford a mortgage and elder care? It takes time to sort through all that stuff. In the meantime, storage units.

For a more scientific answer, ask the clerk that works there, what the average customer's using them for. Probably all kinds of reasons. The housing insecurity aspect is particularly apparent when you sign up for one, though. A lot of emphasis is placed on how living in the units is forbidden, and how many homeless people this company has had to evict from the facility... It's really quite bleak. Towns have no problem permitting climate-controlled houses for our stuff, but fuck people, am I right? They won't even let them have a tent. You will own nothing and be happy.

As for me, I've only ever had a storage unit for a few months at a time. I kinda doubt there's that many people out there, keeping the same unit for years and years.