I think the sweet spot differs with different anglers.

The sweet spot concept is that there is a degree of rod flexion that just feels "right" for want of a better term. Rod flexion depends on the amount of line out of the rod and how fast that line is accelerated. Different anglers have different casting strokes.

Similarly, some fly rods seem to cast better with some lines. Not all lines feel the same even though the first 30 feet of the identical "weight" fly lines theoretically have the same mass. The matching of fly lines to the fly rod and the matching of rod action to the casting style, is why I believe anglers prefer certain lines and fly rods.