From www.leather-dictionary.com - The Leather Dictionary
In the tannery, primers are used to ensure that the colour layers adhere properly to porous aniline leather. The primer prevents the pigment colour from being "absorbed" by the leather and its adhesive effect ensures the colour sticks to the surface. If the leather absorbs excess colour, it becomes hard and plastic-like, as a result of stiffer binders in the leather colour. Primer prevents this and ensures the colour runs more evenly on the leather by "covering" extremely porous areas.
Structure of the leather colouring and finish.