Storage as a concept is not a new idea. People who have special treasures and trinkets have been looking for ways to keep them safe from the earliest days of mankind and other have been looking for ways to take what they want from people who cannot protect and defend what they have. It may sound like a primitive instinct to protect and keep safe our things, after all it is just stuff but then having certain things buys you status in your community so it has become a natural thing to want what others have.
Storage becomes a part of this equation when you have not got the space to storage your precious objects and items. In modern times, simply throwing away things has become unpopular, holding onto things till they age and become valuable is more sensible with certain items, sentiment can also be a reason to hold onto something and family heirlooms can also be items you want to keep but do not necessarily have the room for. There are many TV shows that deal with antiques, hoarders and storage and they are popular because people want to know what others have and what they are worth. Value can sometime be a relative thing and so can space, sometime storage space itself is of value.
People who need storage do not always need it for personal items, there are people who need professional items stored and in the days of phising, hacking and identity theft it is unthinkable for a company to dispose of sensitive information without following guidelines. But what about the paper you need to keep, signed documents that may not have been scanned onto computer. These would need to be stored until this is possible.
Another category of person who might look into self storage is people who just have too much stuff for the size of their home, a family who want to convert a loft for a bedroom would find they have nowhere to store their winter/summer clothes, seasonal items such as sledges or Christmas decorations, skiing/gym/sports equipment or even furniture. Sometimes people do not use items throughout the year and storing bulky items for a whole year you only use for a month out of the year can be frustrating, especially with thing that have cost a great deal of money.