Many Drupal users and developers are members of groups.drupal.org and will be getting email from the site when people post new messages and replies to threads. Of these people, many probably also like to organise their email in folders, for easy reference in the future.
A common way to do this is by creating email filter rules that look at email subject lines or other headers and perform an action based on what these headers contain.
Email lists usually have a specific header, called List-Id, which (often) uniquely identifies the list an email came from. Many email programs have built-in filter rules for this header and thus allow you to easily create filter rules for mailing lists.
Emails from Drupal Groups - or rather, emails sent by Organic Groups, which is what powers Drupal groups - do not contain this header. That means you can't take advantage of mail program smarts that try to use the List-Id and have to filter on (part of) the subject line instead.
But wait, there is a solution, I made a patch! Issue #1490506 contains a patch that adds a List-Id header to emails sent by the og_notifications sub-module. Sadly it has been languising in the issue queue though.
So, if you'd like to use this feature to help organise email you get from GDO, please go and review the patch :-)