From this article by Felicity Cloake:

Like its blood-based cousins, it was originally designed to help use up odd bits of a pig when that valuable animal was slaughtered, generally in the autumn. Offal and scraps were mixed with cream, eggs, herbs and spices, bulked out by breadcrumbs or cereals (hence the groat variation), and then made into a sausage, and boiled, a measure that would have extended their shelf life considerably before the advent of reliable refrigeration.