Depends on what you are talking about with bench shirt, as they have evolved over they years.
Something like a Titan Fury has a stretchy back, and a ridgid polyester front, with arms sticking straight out. BAsically as you lower the bar, the front of the shirt get stretched and holds some of the energy from the lowering, which it then allows you to release on the way up, basically removing part of hte movement.
The extreme shirts like Gene is wearing are open backed, which allow you to wear the collar down lower. What this does (when combined with a 'super' collar) is that instead of the energy being put through one/two layers of chest panel, the stress is being put throuhg 8-10 layers of the collar (his federation has no limit to the collar thickness) enabling more load to be put through it.
a simple shirt such as the Inzer Blast is basically a tight fitting poly tshirt. Offers support to the shoulders and chest while offering little performance increase. Which is a good thing, as it allows soem people with shoulder injuries to train around the injury without adding to hte problem