Some people like being the only person that can do a particular task. Some even claim that it makes them indispensable and therefore keeps them in a job. This is nonsense and you should actively avoid being the only person that can do any given task.

If you are any good at your job, you will soon have plenty of work to keep you busy. Having to drop it all every time someone needs your special secret skills is not productive. Write a manual or train someone else at the earliest opportunity.

If I find myself in this situation, it is usually because I don’t trust other people to do the task properly. Often that means it is worth investing time to simplify, automate or idiot-proof the process. If the process is so complex or fragile that I can’t trust anyone else to touch it, sooner or later I’ll probably screw it up too.