Brie and grape is the cheese board reduced to a sandwich, and that is the most useful way to understand what it is doing. A cheese board offers a soft white cheese, sweet fruit, and bread as three separate things you assemble yourself at the table. This sandwich makes that assembly in advance and seals it: the brie, the halved grapes, and the bread that would otherwise sit beside them are bound into one item you can pick up and eat without a knife. Everything that makes it work is already a decision someone made before it reached the chiller, which is exactly why it has become a fixture of the deli counter and the catered tea tray rather than something people build at home.
The craft, on a counter that has to hold a sandwich for hours, is keeping the brie and the grape from defeating the bread before it is sold. Grapes are halved and packed skin-out so they bleed as little sugar as possible; the brie is sliced into a continuous fat layer that acts as a barrier between the wet fruit and the crumb; butter to the edges waterproofs the bread from the other face and supplies the salt the mild cheese cannot. The bread is plain soft white because the filling is delicate and any assertive loaf would dominate it, and because a sandwich engineered to survive a refrigerated display cannot also be carrying a crust that toughens as it sits. It is built to be eaten standing up, in two bites, between other things, which shapes every choice in it.
The variations are the rest of the soft-cheese-and-fruit counter. Brie with cranberry is the Christmas-season reading, sharper and jammier; brie with fig is denser and more confected; the grape can be swapped for thin apple or pear to bring acid and crunch in place of the cool burst; a few walnuts added give the bitter, structural counter the plain version deliberately leaves out. Each of those is its own balance and its own article rather than being crowded in here.