Knowledge dispels the darkness of ignorance, while light removes physical darkness. The two are associated through an intelligible effect that listeners can readily grasp. Calling knowledge “light” therefore presents its benefit in a perceptible image rather than giving a new dictionary definition.
Where the two concepts are especially close in the intended effect, direct metaphor can carry greater emphasis than merely saying “knowledge is like light.”
The resemblance does not make the concepts identical in nature; it points to their shared power to disclose and clarify.