Knowledge gained through hard work or experience are meant to be shared. You might think that holding on to this knowledge would make you indispensable, or by sharing, it would jeopardize your position since someone else can do it.
Yet, have you considered the possibility that you have limited yourself to the scope of this work? You’ll end up being the person doing it all the time, and losing the chance to gain new knowledge and experience.
On the other hand, if the same knowledge was being shared and passed along, you would have achieved a few things with a single stroke - you would have trained some possible successors that can take over the work, and you have freed yourself to work on and experience something new.