Earlier quoted context omitted.
Some reasons off the top of my head: * I can scribble group meeting notes and share them in Slack by just scanning from my phone and they come out great—including drawings * I can scan my notes quickly as PDFs and keep them for years to come * Some classes allow me to upload and refer to my notes during exams * I don't waste any paper
Points 1,2,3 apply to paper notebooks, too. As for wasting paper, even in my furious note taking days in college, it didn't amount to that much paper. I don't think the stack after graduating reached a foot high. You'll waste orders of magnitude more paper throwing away junk mail, food packages, etc. Besides, paper recycles easily. Plastic rocketbook pages, nope.
Additionally, when I was in my undergrad I still went through a lot of paper. I have bad habits when I scribble notes or solve math problems. But with this, it's the only notebook I carry.
I don't have separate notebooks for work, subjects, home, etc.