Sieve is used to filter messages directly on the imap server. In this way, it can replace local mail filter rules in the user’s email agent. This is very useful, if more then one email agent is used – e.g. if the same server is used from different computers or operatint systems. In my case, the company notebook is Windows driven (using Thunderbird) and my workstation is running on linux using Evolution as a mail client. My mails are filtered by Sieve so I do not need to edit filter rules for each client.