Bill Tindall, E.Tn.
Slips are an English furniture invention to deal with the problems of the thin white oak sides commonly used in English Period Furniture. They have become a fashion statement for English furniture in modern times. I find them useful in my work that isn't English and does not use thin white oak sides.
They provide additional bearing surface for thin drawer sides in any kind of wood. They provide a groove for the drawer bottom in a thin drawer side. And, they make aligning the drawers bottom slots trivial.
The picture shows how I install them, using an alignment piece the thickness of the drawer bottom . After they are glued on the bottom of the slip is planed level with the drawer side. I always have scrap from making drawer sides to make the slips from so no additional wood is required.