One valid criticism of using paper for notes is that searching through notebooks is rather difficult. With my poor handwriting, scanning for certain information in a wash of squiggly lines can be painfully slow.
For a couple of months now I’ve been going back through my notes periodically and highlighting key words and phrases. I’ve found that if I emphasize the most relevant bit of each note, I can find most things fairly quickly. It also helps when simply perusing old notes. Usually, I want to skip anything “meta” like which pen I’m using or the regular “Why am I still using paper?” fluff. Zipping over the highlighted phrases makes quick work of it.
Someday this could also help with building an index. If I ever decide to bother, that is.
So yeah, highlighting my paper notes is useful and I recommend it. I wish I’d thought of it sooner.