The base of any productivity system is basically a list of stuff that needs doing and then going through that list. The ability to, at any moment, rapidly capture some item or idea is fundamental to maintaining a clear mind - without it, your mind will essentially keep remdinding you not to forget this important thing. (For more on productivity, check out Productive Agility.)
For years I leveraged pen and paper notebooks, nothing fancy - just a composition book and a pen. I tried multiple times to convert to a digital method of some type. It wasn’t until the reMarkable e-ink tablet that I finally started thinking I had found something.