One of the reasons why I use Gnus, is the ability to read mailing lists in digest form very easily.
Lots of mailing lists have the option of sending digests of all the mails sent that day, in a couple of mails (say usually under 64K) per day. This is better than having your mailbox inundated with hundreds of emails from the list. And delays the latest Virus/Bot attack too. :)
A digest mail is no different from another email, so most email clients treat it as another article or mail message. But if you open it, it's quite unappealing to read it. One huge text file, with mails separated by a newline. You'd have to scroll up to the TOC (generated from the Subject lines) to get the context and follows up in order to read. Especially on interesting mail threads, it's very irritating.
And to repeat it's all text. Imagine trying to read 64K chunks of plain text. I get around 8 in day on the sas-l list.
This is where Gnus shines. If you open the mail, Gnus opens it normally. Then hit C-d, Tada!
it's now splits the digest as individual mails showing each email by date and author. Just like you'd see any normal mail. And to boot, if you hit C-M-t, it threads the same digest to the extent possible(I mean, some mail threads can span digests, can't it?)
Hitting C-d is the only extra work involved.
This feature alone is the biggest time saver in terms of reading mailing lists for me. A quick glance at the threaded subjects of the digest and I'm ready to either read a few or simply exit the temporary buffer created.
Gnus is awesome in this regard.