Sharing knowledge of a subject you are familiar with is fulfilling. You feel accomplished when you can teach or lead someone. People are grateful when you help them.
Mentoring keeps you accountable and deepens your understanding of the subject matter. Your own skills and comprehension will improve as a result.
Mentoring helps to expose flaws in your own understanding or processes. Mentees often share their outside perspective, which can reveal issues that complacency and familiarity might shield you from.
In some cases, mentoring enables you to delegate work. A rewarding part of mentoring is watching your mentee grow and become self-sufficient. You can benefit by carving out a portion of your work to use as a teaching tool.