I think he sacrificed himself to stop Bison.
It didn't do anything.
Yes it did. The story browbeats the fact that he can siphon away psycho energy, among any energy really, because of his gem. They had like a five minute scene with him and Abel, with the latter going berserk and Nash just dealing with it by removing the P energy from him.
Then, later, Bison gets all that energy, and Nash basically suicides to suck out all the excess (tee hee) and it's the only reason Ryu is able to finish Bison off. Even before Ryu hops up to fight, Guile and Chun commentate about Bison's power feeling different after Nash's suckage.
For all the shit the plot gets, rightfully so usually, it's weird this point gets brought up when the storyline makes it painfully clear what his purpose was. But it's not like the story was super engaging so I can't blame people for turning off their brain.