This technique enables the use of a liner glaze, in this case GA6-B. These are most valuable where the outer glaze is reactive (melt-mobile, crystallizing or heavily pigmented) and therefore potentially leachable. Not only are liner glazes less likely to leach but they are less likely to craze, this assures water tightness and eliminates any potential for bacteria growth in the cracks (especially if the body has porosity). Liner glazes are also less likely to stain and cutlery mark, adding to the durability of pieces. The straightness of the dividing line is affected by the degree to which the two glazes bleed into each other. Liner glazing also adds a decorative element to pieces. This technique is also practical where mug walls are thin and cannot absorb enough water to dry the glaze quickly after dipping or brushing.
Liner-glazing is a very good way to assure that your ware has a durable and leach resistant surface. It also signals customers that you care about this.
How to apply inside and outside glazes to a pottery mug and get them to meet at a clean line at the rim.