Wisdom Tooth are the most largest mollar teeth. 
If wisdom tooth has fully errupted in the mouth and found the right positioning, may not need to be extracted.
If it is in the wrong positioning, wisdom tooh needs to be extracted since they are located in the back side of the mouth and having cavity risk.
Food remains can easily be accumulated in half errected wisdom tooth’s gingiva and cause infections time to time (pericoronal) , thus patienst can not fully open their mouth.  Especially, in  these type cases, patients wisdom tooth can be extracted only if antibiotics treatment is completed.
Wisdom tooth can also be extracted, if the patient does not have enough space for the wisdom tooth and for the possibility of jamming other teeth.
Un-errected  wisdom tooth can also cause cyst formation and rarely these cyst structures can be transformed to mouth tumors.  In these type cases, wisdom tooth must be extracted.