They've increasingly added features to tell you who on your mailing list to advertise to, how (they also handle Facebook and Google ads, some postal mail, etc.), and when.

They can estimate how likely addresses are to lead to spam complaints (https://mailchimp.com/help/about-omnivore/), how likely customers are to spend more (https://mailchimp.com/help/about-customer-lifetime-value-and...), things like that.