The inside of a car wears faster than the outside — sun through the glass, body oils in the leather and everything that gets spilled on a school run.
Interior coatings work on the same principle as paint coatings: they seal a porous surface so nothing soaks in. On leather that means body oils and denim dye stop staining the bolsters. On fabric it means coffee sits on top long enough to be blotted up. On plastics and dash tops it means UV does not chalk and crack them.
The most common damage we see is dark denim dye transferred into light leather bolsters, and cracked, faded dash tops from parking in full sun. Both are largely preventable and much cheaper to protect against than to repair.