A mentor is an experienced developer who works with your staff. There are many scenarios where a mentor can be very useful:
A project needs help with architectural or UI design issues.
Your in-house people are not up to speed in a required technology. They need a boost to get the project started, in the right direction with a strong foundation.
You have an existing application or project which needs work - new features, broken features, better performance, whatever. Your in-house staff would like to be able to maintain it but don't know how best to do so.
Your in-house staff would benefit from another set of experienced eyes.