Well, a good answer would be "because s/he is familiar with codebases A, B and C and has skills X, Y, Z", and I would expect the employer to follow-up with a ra

Well, a good answer would be "because s/he is familiar with codebases A, B and C and has skills X, Y, Z", and I would expect the employer to follow-up with a raise if the employee in question also learns all of the above.