It is a feat in itself to be an expert in your own field, but the ability to pass that knowledge to others effectively is an even greater one. Employers are interested in seeing whether your knowledge can extend to others, given that you will almost always have to collaborate with others in a typical work environment. Being able to give a specific example in which you were able to successfully teach your skills to others reinforces the idea that you are both extremely knowledgeable and that your contributions will not be confined to what you, as one person, contributes to the organization/company, but will also encompass those of others who have learned from you.