What was the Berbers' role in the Sahara
The Berber were a people who lived in and around the Sahara Desert since prehistoric times. Therefore, it is a little difficult to understand what you mean by their "role" in that area.
One way to answer this question would be to say that their role was the role of trader and carrier of culture between North Africa and Sub-Saharan Africa. The Berbers were instrumental in the Trans-Saharan trade that ended up bringing West Africa into contact with the Muslim cultures of North Africa.
The Berbers were also important in helping to bring Umayyad (since you asked about them before) rule to North Africa. Berber converts to Islam helped the Umayyads conquer in that area.
These are the two most important roles of the Berbers in this area in historical terms.