Equations are a shitload faster to write: this is the main reason.
No fucking spell correction.
Every piece of writing is visually unique and looking back at handwritten notes brings back an additional layer of memory (diaries/personal journals eg).
Paper notes can be permanently destroyed with a high level of confidence and low cost if desired.
Written notes can be easily left for/handed to another person (for flirtatious purposes e.g.), or placed semi-permanently in a useful spot.
Electronic notes are great too, I keep my grocery list and whatnot on my phone, but the reasons above are why I also write things by hand.