Used stamps are of great importance to stamp collectors throughout the world. Even though a majority of people who gather stamps as a hobby prefer ones in mint condition, used stamps are also highly desirable, especially if they're rare. There are online stamp dealers that purchase used stamps at a fixed cost and then put them up on display for sale.
Stamp collecting used to be a very prevalent hobby in the mid 1900s. People were very fond of gathering stamps from all over the world and then pasting them on to beautiful diaries and registers. Even though the number of stamp collectors has significantly shrunk within the last few decades, there are still people who'd go to great lengths for maintaining a collection.
Generally, when a stamp series is launched, only a few are left untouched,while the rest are used on envelopes and packages. It isn't possible for all collectors to get their own mint versions of every stamp in the world; hence, a lot of them would happily settle for a used stamp just for the sake of completing their collection.
There are also charity organizations that accept used stamps as donations. They sell them to dealers and make money for their operations.