Why are there mob spawners in Minecraft if mobs (hostile/friendly/neutral) can spawn without them? if the mobs can spawn without the spawner than what is the point of them?
Mob spawners are used to designate "dungeon" areas. While hostile mobs can certainly spawn without spawners, the spawners guarantee the appearance of certain mobs in certain areas. For example, the cave spider nest is a great example. Normally, cave spiders can't spawn randomly, but cave spider spawners "force" those spawns in the abandoned mines where they live.
If there is a spawner under ground, it is likely there is a chest behind it. The mobs that spawn there are just to protect it, I think. And to make the game harder :)
Yawus is right. Also, mob spawners can be used by map makers to achieve the same effect (although I prefer to use a dispenser with an infinite stack of spawn eggs).