Self storage is the epitome of American excess. We bought so much crap at the big box retailers that we ran out of space in our McMansions to keep it all. So, we put some of it in storage. This excess stuff that we obviously don't need, since we haven't used it in years, will stay there in storage, being air conditioned in the summer, heated in the winter, and having rent paid for it, until we eventually run out of space and need a bigger storage unit.
There should be an episode of Hoarders where they find crazy people that have been storing junk in self-storage facilities for decades and look at what exactly they have that is so important that they need to lock it up for years and never see it or touch it.
That said, self-storage isn't just for people who can't stop accumulating stuff. In cities like San Francisco and especially Manhattan, it often makes sense to put rarely-used or not-yet-used items in storage, like camping gear, or heirloom furniture that you don't have the space for at the moment.
I've found self-storage useful when moving between cities -- it's hard to predict exactly when you'll find the right place, so it's a reasonably cheap way to buffer the transfer.
There should be an episode of Hoarders where they find crazy people that have been storing junk in self-storage facilities for decades and look at what exactly they have that is so important that they need to lock it up for years and never see it or touch it.