I think in part it’s that this seems highly un-mysterious & easily retrievable. Lacking universal expectations for how things might be attached and retrieved together in the spatial desktop environment, this is very clear: my note is on the back of the webpage. It won’t get lost, (...)

(...) and I’ll always know how to get to it. I think another reason is that it’s a bit of a delightful brain breaker. “Wow, I’ve never thought about what’s on the back of a webpage or window before. Now I know: my own notes!”