I get scammers using my email to sign up for websites, but they very obviously cannot login to my account. I often wonder what is in it for them. I'm sure someone on HN can tell me!
I assume they just need a valid email address to put in and hope they can use the service without email verification.
Commission schemes, possibly. Sign up with their code and they get something out of it. So they submit 10000 harvested addresses, and hope some small % of them think it's something they signed up for and complete the registration process.
You'd think big companies would know better than enlisting spammers to spam on their behalf, but I'm pretty sure Netflix had a scheme like this a few years ago. "Grow at any costs" sites like streaming or social media are probably happy with a tiny bit of plausible deniability for their spamming.