You can think of them as the matrix equivalent of a drone.
That use of an agent would essentially make it a form of IC.
It can also be used as a hacker's aid, healing him or repairing his programs with the appropriate program during a fight, tracing an escaping enemy while the hacker works at something else, etc etc.
I expect that the most common use for a low to moderate rating agent is data-mining. Load it up with a browse and a stealth program and set up some search parameters and let it go. While you're going about your day doing whatever, it is tracking down, collecting, and collating data on the subject of your choice. Come back a day or two later and you'll have a ton of collected, verified(ish), and ordered information on the subject of your choice ready to peruse.
It's efficient for that sort of stuff because extended tests only care about total number of hits and whether you glitch or not. The Agent can work 24/7, so even a lower to moderate rating agent can do as well as a skilled tech-head who needs to sleep, eat, and worry about meatspace stuff to some degree.