tl;dr; version:

While there are good use cases -

> The United States is the world leader, with roughly ninety per cent of global capacity. [...] there are more self-storage facilities in the U.S. than there are Starbucks, McDonald’s, Walmart, Home Depot, Domino’s, Dunkin’, and Costco locations combined. Annual revenues are estimated to be more than forty billion dollars.

> I asked a Welsh facility manager I met in Las Vegas what his customers keep in their units, and he said, “It’s . . . it’s . . . rubbish.”

> My wife and I have a friend whose mother died in 2006. She and her siblings decided to sell their mother’s furniture and donate the proceeds to a charity that she would have approved of. “We put everything in storage temporarily, and it’s all still there,” our friend told me. She figures that they’ve spent at least fifty thousand dollars in rental fees so far—more than they could get from selling the furniture.

My mother-in-law could have been a case study for this article too. A couple decades ago she was short on cash and long on wanderlust, and her kids were grown so she packed her stuff (mainly unexceptional furniture) into a storage unit, got a cheap 1 bedroom apartment in a new city and made a new life. A decade later, she decided to try in another new city, with another tiny apartment.

All the while paying for the storage unit, occasionally talking about the cherished "family heirlooms" therein; those things she'd bring out of storage when she got a bigger place. Especially a china cabinet she had to fight her sister for when her mother died.

Now she's having health problems and is approaching the end of her life. I'm sure she'll keep paying for the storage unit until the end but soon enough the day will come my wife and I have to travel several states away to empty the contents of the storage unit into a dumpster. Including the "beloved" china cabinet that her kids have already decided they have zero interest in (and probably smells of mildew and mouse urine at this point).

I don't know the total amount spent on the storage unit over the years but I've got to think it's in the 5 figures. Enough to pay for a car, or some nice trips, or ... to keep old bedframes in a big closet.

People can get unusually attached to their stuff regardless of its objective value.

> She figures that they’ve spent at least fifty thousand dollars in rental fees so far—more than they could get from selling the furniture.

That probably doesn't even consider inflation, $1 in 2006 was worth $1.60 in today's money. People are really bad at long term financial planning but also get really attached to objects with some sentimental value. So this is a market where it's even easier to exploit that poor financial education.