How often do you put pen to paper?

Math will always be difficult to 'computerize' - just the way I like it.

IMHO, writing short notes by hand seems to act as a mnemonic device - it installs stuff in memory much better than typing.

I also find myself sketching 1st drafts of network diagrams/dataflows with a soft-leaded sketching pencil. Somehow this clarifies my thinking so that once I create a proper digital document the process goes much more easily.

I solve problems for a living. Rule #1 for problem solving is "Draw a Picture". Much easier to do that in my engineering notebook.

Any sort of mathematical derivation is easier to do on paper. Initial storyboarding is easier on paper (before you whiteboard it in front of a group). Diagramming data flow. The list goes on...

Sure, there are tools for putting together pretty presentation-worthy versions of these, but for rapid idea sorting throughout the day, pen and paper is the only way to go.

You don't use TeX for doing the calculations, you use a language like R, Mathlab, or whatever, depending on the kind of mathematics you're doing. Doing calculations by hand is something you do at school to prove to yourself that you really understand what's going on. If you're doing it for real problems, rather than giving instructions to a computer, then you're doing it wrong. There's a reason why the computer science and mathematics departments generally have very close ties in most universities...

I seriously believe most of those pretty presentation-worthy versions should be hand-drawn, too. It's faster to draw by hand, and the results are more readable than what you get by connecting boxes in MS Powerpoint, MS Word, etc. Then you just take a photo, adjust brightness/contrast, and paste it into the document.

Not for things which are going to be read by people in suits perhaps, but most diagrams I make are only read by other programmers.