The common reason for antogonistic supersets is to take advantage of reciprocal innervation.
This is an effect that shuts off (or down) the opposite muscle group to allow another to work. Your pecs and triceps are firing in the bench press, so your back and biceps must relax. Then, you go to a row, which uses your back and biceps. This forces your pecs and tris to relax.
However, this "forced" relaxation allows you to recover faster than you would just sitting there and relaxing. You can either shorten you workout time, or lift heavier, depending on how you structure the timing of the superset.
No supersets
Set 1 of bench press, 2 minutes rest
Set 2 of bench press, 2 minutes rest
Set 3 of bench press, 2 minutes rest
Set 1 of row, 2 minutes rest
Set 2 of row, 2 minutes rest
Set 3 of row
10 minutes of rest, plus time to change exercises (1 minute?), plus lifting time
Superset 1 of bench press and row, 2 minutes rest
Superset 2 of bench press and row, 2 minutes rest
Superset 3 of bench press and row
4 minutes of rest time, plus time to move between exercises (2-4 minutes?), plus lifting time
In this example, the chest actually gets more than 2 minutes rest while you do your rows, so you can lift a little heavier and save time. Or, shorten the rests a bit more if you don't want to get too huge.
So, we're talking a minor time saving, in the example (3 minutes, maybe), but you also lift heavier in that example. Bonus.