Mainly to save on the micro mgt of the smaller ones. 6 Gives me 2 for food, one for defense barracks, one big ass cannon and 2 for supply routes to castle and sanct. So long as they stay happy and man the cannon, job done. I tend to put up at least one missile turret and a lot of machine guns ones. I have a kinda template building I put up, concrete for the outside walls, plants and water on the inside so they dont have to leave the building, makes it easier to defend. Assign them all to beds in that structure so they dont go wandering off and leave the door open
Sanct & castle are at 19 settlers and that starts to get a bit micro-ish when you add stalls, scavengers, etc. I have tried deckin me out in different outfits so I can at least tell whose doing what, but even that was painful with 2 large settlements.
My Sanct settlement is a massive set of structures now, takes a while with no mods, but I literally scrap everything in each new settlement to 'fund' building Sanctopolus up. I will get some screen shots when its close to finished, but thats still a long way off