A list of emails. That's essentialls a database query that is taking X items and sorting by the date field, most commonly, except that the average user can have thousands, or even tens or hundreds of thousands of items that are unique to them in that dataset that need to be sorted and returned.

Sure, gmail optimizes for this heavily so it's fast, but it's still one of the most intensive things you can do for an app like that, so reducing the amount of times you need to do that is a huge win for any webmail. If you've ever used a webmail client that's essentially just an IMAP client for a regular IMAP account, you'll note that if you open a large inbox or folder it's WAY slower than trying to view an individual message, most times, for what are obvious reasons of you just think of a mailbox as a database of email and the operations that need to happen on that database (which it is).

If clicking on an individual message is a new page, that's fine, but if going back to the main view is another full IMAP inbox query, that's incredibly resource intensive compared to having it cached in the client already (even if the second request is cached in the server, it's still far more wasteful than not having to request it again).