‘Ink’ may not be exactly the right word: perhaps ‘paint?’ Basically, there are several classes of spells (divination, warding, and exorcism) which work noticeably better if the sigils and lines used to express the spell physically are drawn or written in Applesauce Ink. There aren’t any spells out there that require Applesauce Ink, and the bonus to cast isn’t exactly obvious: it’s more like that using Applesauce Ink helps make up deficiencies elsewhere. If a spellcaster is in a hurry, or working with substandard materials, or under stressful conditions, well, an apple a day keeps spell failures away.
Note that making Applesauce Ink is fairly trivial (applesauce and food coloring), as long as you have the right kind of applesauce. Naturally, that applesauce is not commercially available — no, wait, it is. OK, it can only be purchased by people with the right kind of connections; sorry, that’s not true, either. Maybe it’s one of those special kinds of organic applesauce that lives in the tedious aisle of the supermarket? No? It’s really one of the top national brands, you can find it anywhere in the USA, the stuff is processed normally? Yeah. Apparently absolutely nobody not already in the know ever thinks to look in commonly accessible places when they’re trying to track down magical ingredients. It’s the ultimate hiding in plain sight, really.