So it seems to me, that all of the Commanders of the Revolutionary Army have a clear purpose in the organization. Although none of this is outright confirmed it does make a lot of sense to me, so here goes:
#BELO BETTY: Front-woman and Leadership
Although the commanders seem to have their own territories and are supposedly scattered most of the time, Belo Betty had the say in this situation. She told the other commanders what to do and it was her who addressed the civilians and encouraged them to fight (obviously enhanced through her Devil Fruit powers, but it seems to me Oda is making her out as a charismatic person either way). Her Power seems VERY fitting for a group of Rebels and explains some of the Revolutionary Army’s success in swaying the civilians.
#MORLEY: Infiltration and Retreat
We don’t have the name of his Devil Fruit but many speculated that his ability would be very well suited to dig tunnels for the Revolutionary Army to travel quickly, or at least stealthily from one location to another. Many speculated also that it could have been him, who built level 5.5 in Impel Down.
#KARASU: Transportation and Communication
His Ability is very likely connected to the Crows that transported the Revolutionary Army members off of Dressrosa (Notice One of the crows actually talking to Sabo) and delivering the 3D2Y newspaper to Robin.
So it seems he’s an inventor. Definitely useful in almost any crew or scenario, however it’s tough to speculate on the extent of his purpose in the Revolutionary Army. Is he just inventing weapons for himself? For the whole Revolutionary Army? For helpless civilians who want to stand up against a tyrant?
*Theory by CabbageTheVoice