What is the difference between fudge recipes that call for sweetened, condensed milk versus regular milk?
Fudge is basically a chocolate flavored candy.
You combine chocolate and a sugar syrup and boil the syrup until you get the desired concentration. Everything else about the recipe is to either minimize sugar crystal size or add flavor.
Sweetened condensed milk will do two things-
1- give you a nice, caramelly flavor
2- Save a little time. Since it has less water to begin with there will be less to boil off.