Your full-sized activity object might only show a few first rows in the view window, but piles of email data are taking a toll on your Salesforce data storage.
In Salesforce, each email received from the customer’s end is reflected in the ‘Email’ tab from the ‘Activity’ object. If the user assigns Email-to-Case, Salesforce logs into the customer emails to automatically transform them into cases or other associated objects in the backend database.
Read more https://www.dataarchiva.com/how-archiving-old-salesforce-email-improves-storage-efficiency/