They create the surround in surround sound. There are basically two versions. One is the small satellite which need to work with other speakers to produce meaningful sound....
such as a subwoofer and centre speaker (the one on the subwoofer) or they are smaller speakers which add on to a more traditional setup to give the surround sound...........
such as the two smaller speakers here, which sometimes can be used on their own as they are traditional bookshelf speakers in their own right.