Or a person could look at their skills, come up with a rotation based on that and not need a parser at all because they can tell how well they are playing based on their rotation.

I can't speak for everyone of course but I find it quite easy to come up with a rotation just by looking at my skills and not using any outside help to do so. Last time I decided to compare what I came up with, with a rotation a person who uses a parser came up with, it was the same freaking thing.

No ingame tools or third party tools needed other than your own brain.