http://www.aim.com/other/quick_buddy.adp
Along with AIM Express, QuickBuddy allows you to log on from a remote machine, such as a lab machine or a box in just about any office, and access their Instant Messaging database to send an IM to any of the other AIM or AOL users.
Though definitely not as 'advanced' as the AIM client (if the term applies to it), the ability to log on in a remote location or one that does not have the AIM or AOL software installed allows you to reach someone quickly. This is primarily useful in the case you need to reach someone else to give or receive time-sensitive information, or alert them to an Email you've sent, and you know said person will be on AIM.