Fetchmail and sendmail are useful if you have multiple users on your system. That way, a single process can retrieve the email at regular intervals for all users on your system, no matter what email software they are using (Evolution, Thunderbird, pine, mutt, etc.).
Using sendmail witth spamassasin, all your mail will get filtered for spam before it gets to Evolution. Evolution can interact with spamassassin (using the Junk and Not Junk buttons), to adjust the spamassassin filters.
Using fetchmail, your mail is retrieved and stored locally (on your system), even when you are not running Evolution (if the system is up). That's handy if your ISP provides limited email storage, and you get a bunch quickly (spam, email with large attachments, etc.). If you're not retrieving email fast enough, many ISPs will either start rejecting email, or worse, discard it.