If you manage a database externally from MailChimp, you don’t want to keep creating new lists in MailChimp every time you want to send a new campaign. Tons of lists get hard to manage.
Instead, sometimes you just want to log in to MailChimp, copy-paste your most up-to-date version of your list, and then have MailChimp auto-detect whether or not there are any changes to a subscriber’s profile.
So whenever you import your list, check this box in order to do that auto-update:
Hint: this is an extremely powerful option if you’re sync’ing your database with MailChimp via our API. For each campaign you send, you can automatically update your customers’ data (like purchase history, product alerts, etc), then use our advanced merge tags in your campaign to insert dynamic content for each recipient. If you take a moment to think about that, you’ll actually poop your pants, so be careful.