What is the point of a strong upper body?

I mean you see guys like maurice/bj and most other guys who ran sub 10. They all have great upper body development. But how specifically does having a strong explosive and developed upper body help a sprinter? Increase their stride length? How does having a huge bench press help ben johnson or bruny surin? What would happen if a sprinter completely neglected their upper body weight training and plyometrics?

Well for starters don’t forget that to work the legs you need to load via the upper body. When performing the Olympic lifts a major limiting factor is the ability to support a weight hanging off of the arms via the back.

also, sprinting starts from the arms pistoning. the mid section needs to support and stabilise, mid section is more than abs, its lower back too. to develop lower back, upperbody exercises are needed too.
think of the arms pistoning back and forth, and think of the exercises you do that support that movement.
think of your mid section and exercises that help develop that.
then you will have your answer.