Most poker players who get past being rank novices soon learn how to count their outs, and memorize a few frequently encountered probabilities. However, most still remain stymied when faced with more complex poker math situations, or ones that can’t easily be looked up in a book or online.
Heck, I have on my shelf a whole book filled with tables of poker probabilities (Texas Hold’Em Odds and Probabilities by Matthew Hilger — a really useful resource), but at least a few times a year I still come across poker probability questions that I can’t look up there or anywhere else.