Every team has a job only one person does properly. This is how that job stops being a person and starts being part of the system.
A description of a process is always missing the decisions the person making them stopped noticing years ago. The skill asks about those: what happens in the edge case, which field matters, what makes an output wrong. Then it writes.
Two people running the same skill should get results in the same format. That is what makes the output reviewable, comparable week to week, and safe to put in front of a client. A prompt cannot promise that; a skill file can.
Every team has one job that only one person does properly, and that person is a bottleneck and a risk. This is how that job stops being a person and becomes part of the system.
Skills compound. These are the ones we usually install alongside it.