Most recently, my written notes have been marked up in Markdown too.
The reason for this is simple: Markdown is so easy to use, so easy to read and so easy to implement that it was a natural progression to transfer the system to analog notes. Obviously, analog notes aren’t actually styled with italicized and bold letters. But the small syntax additions help add context that is otherwise hard to convey.
I often do this as well. Especially to denote italics and bold.