You can use the MIDI out to control anything that has a MIDI jack. As well as synths and drum machines, you could also use it to control a mixer or effects unit that has MIDI capability.
At one point around 1990 or so, and I never saw one in real life, Roland had a MIDI controlled 6 outlet ac strip if you can believe that. I guess you could use it on stage and have stuff like a popcorn machine start at measure 142 or whatever else you could think of.