If they are particularly unscrupulous, there is an unsubscribe link: they capture use of this link to validate emails, then aggregate and sell that information onto other spammers.

WAG: I think in general people think building any kind of email distribution list can be monetized, even if it's not being done immediately. Either sold off wholesale or directly monetized through adverts / affiliate links.

Sounds like a university freshman’s lemonade business gone wrong