Geeknamese
Explorer
Inside the Fog Cloud, the mooks are at disadvantage to hit the PCs because they cannot see them. However, the PCs cannot see the mooks either, so the chances to hit a blind creature has advantage. Advantage and disadvantage cancel out. Why cast the spell? It does nothing in this scenario other than allowing the spell casters to cast "to hit" spells without disadvantage.
Another example, again, the wizard casts it to protect the PCs. Enemy archers try to attack from outside. Again, advantage and disadvantage cancel out, at least on round one where the NPCs knew which squares the PCs were in.
As a DM, I would say that attacks against Blinded creatures only gain Advantage if the attacker can see themselves. If both of you are in a Fog Cloud and neither could see, you're both at Disadvantage on attacks in my game. For me, it's pretty clear that that was the intention of the Blinded condition - for the attacker that has an advantage to receive an advantage on the roll.