Especially in the context of game dev! In path of exile, there was a league years ago called Breach. Your character would run into a Breach and an expanding aura will glow around it, then monsters started spawning one after another.
On the backend, crystal makes that super easy so you don’t need to fiddle with your main game loop, add variables to track a counter/timer, etc. In my case, I just do
spawn start_cosmic_bloom(zone, map_entity) and inside that, have a loop with sleep xx to send the monsters. Doesn’t block the IO stream, and even better, we can pass arguments to it… just incredibly easy!
Another case where you can use them is when doing something similar to Diablo 2’s Baal Run (the boss spawns waves of enemies). Although I have my own class for that because there’s a lot going on but it’s a similar concept
When creating anything on the backend that needs to happen sequentially, they are just fantastic! There are probably countless other things they can be used for too… :O