I teach comics classes in Ann Arbor, MI, and we make a ton of mini-comics in class. Not only are they fun to do, but the implied limitations in length makes it a much more manageable project than an ongoing webcomic. You get the positive feedback of "finishing a thing" in a relatively shorter time, which is so valuable in keeping motivated in your work.
As others have pointed out on this thread, you also then have inexpensive marketing materials to take to shows. I bring mini-comics to every convention I go to and sell them along side my collected series and graphic novels. Provides that "Good/Better/Best" product price range everyone advises you to have at your table.
One thing that I've found is helpful in enhancing the assumed value of my mini-comics is printing the covers with a silk screen. Something about the tactile feel of the ink on the paper, or the sense that a person actually pulled the prints on the covers, changes the buyer's assumption from "inexpensive marketing materials" to "art object."
It doesn't have to be anything fancy, either. I did a fairly simple design for my Boulder and Fleet: Adventurers for Hire mini-comic.
You can get a Speedball screen printing starter kit for under $40. This is what I got started on. We used our bathroom as a dark room for burning the screen, then just pulled the prints on our dining room table. It can be messy, but it's also a lot of fun to do. And you can make limited run art prints for conventions with it as well!
