The saying "knowledge is power" is overused to the point of being cliché. But like most clichés, it has a nugget of truth to it. In a world of magic, where threats can be anything from mundane brigands to psionic half-dragon medusa priests of elder gods, it holds especially true. Divination spells allow characters to learn all manner of useful information: whether something is evil, the location of the special key, the presence and strength of magical auras, the presence of invisible creatures or items, and much more. And magic isn't the only source of crucial information. Knowledge skills allow characters of all classes to have the knowhow to solve problems. At high levels, Knowledge skills themselves can seem almost magical, with characters possessing fine points on the most minutiae of details.
The product you hold in your hands (metaphorically speaking) grants all manner of new abilities to those who seek knowledge, whether through magic or mundane learning.