Footrings, as opposed to flat bottomed containers, lift the piece off the table and enable glazing all of the bottom. While foot rings add extra effort to the finishing stage at fabrication, they also make it easier to glaze the ware (articles can be dipped and quickly sponged to remove the glaze). Only shallow foot rings are possible in machine made items whereas hand made pieces can distinguish themselves with much deeper rings.
An example of a foot ring in a cone 10 reduction mug (it was tooled and sponged at the leather hard stage). It has channels to drain water in the dish washer.
An example of an unfinished foot ring (on a salt glazed mug). This technique is popular with many potters.