Two reasons I can think of:

• Information is more trustworthy if you can arrive at the same conclusion or at least understand how the other person arrived at it given the same information. If someone tells you that person X should be avoided, you could either blindly follow that advice or ask for a reason to decide whether you'd agree with the assessment.
• Knowing the reasons behind something is also useful for extrapolating to new knowledge. A child who asks why they need to bathe will not just learn to wash themselves regularly but also about hygene in general, societal expectations about body odor and/or a possible disease vector.