When you grow other people, you grow your influence as a leader.
Everyone they influence is getting your secondhand influence. Everyone you successfully grow as a mentor becomes a more capable professional. Your team becomes stronger. Your company becomes more productive. Your clients get better products.
The saying “train your replacement” sometimes gets people worried, since the thought occurs to them that, “If the new person knows how to do my job, what do they need me for?” When you are training your replacement and growing your people in the responsibilities of your current position, you ideally should have a vision of where you want to go when they are ready to take your old job, and be taking the steps to prepare for that. Do you want to grow your department and take on bigger projects? Do you want to move up with a promotion? Are you looking to retire in the coming years?
It would be a real shame to be passed over for that dream-job promotion just because there is no one else capable of doing what you are doing now. Just sayin’.