Shaykh Al Uthaymeen was asked:
What is the benefit of the students of knowledge learning (about) the different sects such the Mu’atazilah, the Jahmiyah, and the Khawaarij, whilst they aren’t present in this time (period)?
He replied: 
There is a benefit in Learning the different innovated sects in this time period and it is:
That we come to know the sources (or that which they are criticized for) of these sects so that we can refute that if they are present, and in reality they are present. 
As for the statement of the questioner: “They have no presence at the current moment” then that is built upon his knowledge.
But that which is known with us and with other than us whom observe the situations of the people that these sects are present and that they are active in spreading their innovation. 
For that reason it’s a must that we learn these opinions so that we know it’s falsehood and that we know truth, and so that we may refute those who argue in regards to the truth. 
Kitabul Ilm 128
Majid Jawed Al Afghanee