Rock bands have a lot of equipment. When you go to a show these days, you are there for more than just the music alone. You want a whole performance. You want to see jets of fire shooting from the stage while the guitarist stands on a stack of amplifiers and plays a howling solo. Even pop acts and country acts tend to be big productions, often featuring dancers and props. One of the lesser-known things that a big band has to have, though, is a caster.
If you want to see some real casters, check out the Hamilton casters at Accesscasters.com. You can also just look on the bottom of any half-stack amplifier, such as the Marshall JCM-900. The lower cab is going to have a set of casters on the bottom, with one on every corner. For a full stack, the upper cab may have casters as well, though they will fit nicely into the slots in the top of the lower cab so that it does not roll away.
These casters are so important because they make it possible for bands to bring all of their gear and set up the stage before each show. If amplifiers were not mobile, the job would be a lot harder, and bands would probably just play using the house equipment. This would take away from that huge show that you came to see, that amazing production with all of its moving parts, so casters really help create modern rock concerts.