As @Warren Denley55 has already noted, many folks who are affiliate marketers use an email list as one of the ways to build income streams.
That said, you might consider if you're coming about things backwards. If you don't know why you want an email list (or how you're going to make money with one), it probably makes sense to take a step back and look at the broader plan or goals for your business.
Who are your ideal clients or customers, and what are you going to provide of value in service to them? If you can answer this question, the use of your email list will likely become clearer.
While I would argue there are very few businesses in 2020 that shouldn't have an email list, trying to monetize the list shouldn't always be a goal. There are many successful businesses that use email without directly selling digital products (either their own or as affiliates) via their list.