In addition, the mere act of giving that information to somebody else.
On a phone I can obviously text somebody, but what if I’m somewhere with bad signal (and yes, those places often exist), or the person doesn’t have the phone in their pocket right that second (yes, this also happens in places with work where people don’t want to risk the phone in their pocket breaking)?
With a mini notepad, I can rip a sheet of notes off and hand that diagram to somebody else. If it’s work that will take some time doing while following a diagram, having a phone screen locking up because it isn’t being touched is a hassle and going into the settings to change it back and forth is annoying.