On the server there are no naturally spawning mob spawners. In order to get mob spawners you can purchase them from the shop in spawn

Spawners can be useful as they do not obey to the mob count on the server, meaning they can keep spawning and the mobs that spawn from them will stack into one entity

You are able to use silk touch on a spawner for a 50% chance of getting it back or using explosives for a 100% chance on getting it back