It's wild to me that a paragraph or 7 of plain English that amounts to "be good at things" is enough to make a material difference in the LLM's performance.
As the base is an auto-regressive model that is capable of generating more or less any kind of text, it kind of makes sense though. It always has the capabilities, but you might want it to emulate a stupid analysis as well. So you're leading in with a text that describes what the rest of the text will be in a pretty real sense.
I read once (so no idea if it is true) that in voice lessons, one of the most effective things you can do to improve people's technique is to tell them to pretend to be an opera singer.