The single-arm dumbbell bench press appears to be an upper body pressing exercise, but it's actually more appropriate to categorize it as a core anti-rotation movement. This is the most challenging aspect of the exercise, after all. The unilateral loading forces the body to resist against rotational forces acting upon it.
This has great carryover to the regular bench press:
It requires a good amount of leg drive to keep the hips stable and allow for force to transfer throughout the body and into the weight.
It requires you to "connect" to the bench by engaging the hips, upper back and the entire trunk to remain flat on the bench.
Due to each arm working independently, it can help even out any side-to-side strength imbalances.
It will make the entire body tremendously strong, which will carry over to literally any other lift.