To be able to send Campaigns and autoresponders to purchasers of a certain product.
"When you use MailChimp's Ecommerce360 feature, we make it possible to track individual visitors to your website from your MailChimp campaigns, capture order information, and pass it all it back to MailChimp"
Ubercart has this integration, Drupal Commerce doesn't.
Ubercart Mailchimp module
Mailchimp API regarding ecommerce