Free will. The idea is that for free will to exist you must be able to choose the wrong action.
If a supreme being rules out all wrong actions or prevented you from taking wrong actions, how could there be free will? How could you even be responsible for your own thoughts and actions. How are you not just a puppet?
Alternately you can think of it as a leveling up. It seems like the Apple is always represented as “knowledge of good and evil”. So originally they’re just animals. They take actions but there is no morality, nothing is good or bad. But if they use their free will to take this one forbidden step, they receive the knowledge of good and evil, they can act good or act bad, they know it’s good or bad, and they have the free will to choose their path. And they are accountable for those choices. Now they’re human