Often writing for yourself (“how I did x when I didn’t know how”) is useful enough without being an expert on the subject. You can refer back to it later on if you forgot how you did it, or someone else who was in your shoes will appreciate it. Also makes things less daunting (“hey others are going through the same as me”).

Everyone learns things in a different order and in different ways, so an alternative explanation or source of information can still be valuable. Sometimes I have an important realization simply because something I'd already known for years was presented from a different angle, or maybe just because I rediscovered it in a more fitting moment of my life.

And if someone doesn't find it useful, well, they'll just stop reading and go somewhere else.