Spawners are NPC (Non-Playable-Character) blocks/spawners that can be bought for 3449 gold pieces which is quite expensive. They can serve many decorative purposes as putting them in a house or around for the player to be less lonely.
NPCs can be used as guards castles, to put in quests people, or just for decoration and other purposes they can be chosen as any of the player skins unlocked and will spawn in the direction the player is faced in. NPC's can be killed by any weapon by attacking their body.
Note: NPCs can't move nor harm anybody, mostly used for decoration or target practice.