Just about the only county functions are sheriff and register of deeds. The sheriff runs the county jails and his biggest responsibility is running a taxi and boarding service for prisoners awaiting trial. Sheriff doesn’t do any significant law enforcement.
Sheriff’s office and registry of deeds are the proverbial hackaramas.
Middlesex County took on the responsibility of running two hospitals which were vital to providing jobs for the relatives and (invariably girl-)friends of county officials who couldn’t be hidden in the sheriff’s office or registry.
Atlanta/Fulton will never do that because too many patronage jobs would be lost. Basically the same problem you've got in Mass, with the govt being run by the pols, of the pols, and for the pols and their girlfriends and relatives. To be absolutely fair, Fulton as currently constituted is made up of THREE counties - the original Fulton in the middle, and Milton and Campbell stuck on the ends after they went bankrupt during the Depression. It looks like a long skinny Gerrymander but really isn't. The outlying areas have always complained about getting short shrift, and recently the folks in the former Milton County took matters into their own hands by incorporating the entire former Milton area outside the long-standing cities of Roswell and Alpharetta into three municipalities - Sandy Springs, Johns Creek, and Milton. Don't know if it's going to help as much as they hope. There is still a movement afoot for the former Milton County to secede from Fulton, but there are constitutional barriers to that.
But in the rest of GA there is a whole lot of rural real estate. Our county, Cobb, is one of the central metro counties, but there is only one good-sized city (Marietta, the county seat) and a number of smaller incorporated municipalities (it seems silly to call them cities when you can walk across them in ten minutes). Most of the county's area is what we refer to as "Unincorporated Cobb". That's of course even more true when you get out into the truly rural counties. My parents live in a rural coastal county, and the only incorporated town in the whole county is the county seat - population 1500.
Massachusetts is a whole different story because it's so much smaller and the overall population density is so much greater. GA is the largest state east of the Mississippi, and it has 159 counties (probably too many from a practical standpoint, but an artifact of the days when a farmer had to be able to drive his wagon to the county seat in a reasonable time.)