Why is the Black Piece on this Mechanism Made of Plastic?

[20GT]

why is the black piece plastic instead of just extending the metal? its bolted down. is it just for the axel support? Can you think of a reason they did this?

thanks for your opinions

 

[Joppy]

I can't imagine how this mechanism works (shoots baseballs?). What happens when you pull the lever?

In other words, what is the primary thing that the plastic component in question does exactly?