Wisdom teeth which are expected to erupt between the ages 17-25 are usually trapped in jaw bone or gum as your mouth has no room for an extra set of molars. 

The biggest problem is partially erupted wisdom teeth. Lap of gum tissue that easily holds food particles and debris and is a hotbed for bacteria which will eventually reach jaw bone. This causes infection, swollen gums and pain. Dislocated wisdom teeth leaning against other molars cause pain in the jaw bone due to pressure. In some cases this pressure causes abrasion and cavity in the healthy tooth. 

Wisdom teeth are difficult to becproperly cleaned because of their position and are prone to decay.

Due to these reasons and considering the possibility of cysts they will cause; wisdom teeth are advised to get removed.