If your customers have multiple locations, and/or you do a lot of business with them, they may have had to deal with an inbox full of apparently identical emails. By default, all estimate emails used to have the same subject. This was also true for invoices. Which could have made it hard to know which estimate was for which job at which location.
Now, when you click the "Email" button on an estimate or invoice, you'll see that the job number is automatically added to the beginning of the subject, and the customer name is added to the end. (Of course, you can still edit the subject before sending.)