Webhooks allow your local application data to stay in sync with data in MailChimp. In essence, enabling webhooks makes your application a two-way communication. The API sends data handled at the site to MailChimp, while webhooks allow MailChimp to send data handled there back to the local site.
The most important place for this to be operable is with subscribe/unsubscribe data. If a user unsubscribes via MailChimp rather than updating their local site profile, webhooks are necessary for MailChimp to notify the site of the user data change so that the site data can reflect that the user is unsubscribed. Otherwise the user’s profile data would be out of sync.
Webhooks can be used for updating the user’s status (subscribe/unsubscribe), their email address and changes, as well as any profile data that is handled in a merge field or an interest field.
To enable receiving webhooks, you need to create a webhook key in the WP-Members > MailChimp tab. This doesn’t have to be a random string, it can be anything really. It will be used to create the endpoint for receiving webhooks from MailChimp.
Once you add the key to the settings and save, you will be provided a callback URL to set up at MailChimp. This is the URL MailChimp will send webhook notifications to.
There is information on enabling webhooks in MailChimp here.