Many mini comics come in different kinds of quality. Some have been hastily put together with just white paper and are of a very low quality. Other's have full color covers and are printed on higher quality paper.
Mini comics are used in many ways. Some use it as a way to get started creating comics, and try to sell them online or at a local comic book store. Others use them as a means of advertising a new work, webcomic, or to just get their name out there. Some comic book professionals use them to have something to sell at a comic book convention, or to showcase a new concept. A good way to get a cross-section of the kinds of mini comics is to visit a comic book convention. You can see all kinds of mini comics there, which may give you some ideas to do for your own mini comic.