What is the point of a 25kg plate that thick?
None really, they're back from when those black training bumpers were new on the scene. For some reason all the #25kg were made that way, and that series of bumpers was so cheap and decent that no one cared. The #25kg were sold cheap after a bit, some places blew them out at $ .50 a pound.
Some guys liked them for overhead work like axle and log press. They're a bit larger than some #45's, so you could mix iron and bumper, and still have the safety of a bumper plate. Most of the time coaches just used them to weigh down things or stacked them and used them as a stool.