There are plenty of different ways that people use sheds, from simple tool storage to full living spaces. I can think of a fair few. For example, you could use it as a games room, an animal shelter for your pets, a wood storage facility, a gym or workout room, a pool changing room, or a storage space.
That being said, I think the main reason to purchase a shed would be to keep your property safe. Rather than leaving tools, bikes, and other outdoor objects outside and open to theft, you could lock them safely in a shed.
I did some research to try to find out what the most popular uses of sheds are by looking at the types of sheds commonly sold. It seems that Kent Sectional a retailer that sell Beachcomber hot tubs, and offers three main types of sheds: standard sheds, sturdy workshops, and potting sheds. Presumably, then, most people use sheds for these three purposes.