It's been a while since I've watched the movie, but I don't think they specifically needed transparent aluminum, but it's how Scotty paid for them getting what

It's been a while since I've watched the movie, but I don't think they specifically needed transparent aluminum, but it's how Scotty paid for them getting what they needed. They didn't have money to pay for it, so this was a barter, the knowledge of how to make it for making them some.