I'm always carrying at least two decks of cards, even while I'm at work - and one of them's usually a stripper deck. I'm ready to perform a handful of tricks at a moment's notice. My wife is supportive of my performing, but can't stand it when I take out a pack of cards at a function. I'm usually practicing lifts, exchanges and card controls, sometimes experimenting a little to see "what if..."
They also come in handy when I'm killing time and have a place where I can play a little solitaire. I mean, I have a solitaire game on my iPhone, but it's not the same as shuffling and dealing cards. My solitaire game of choice is Canfield, followed by Klondike, Pyramid, and a variant of Clock that I can actually play with the entire deck in my hands, no table needed. I only play that, though, when I have no choice - the odds of winning Clock are less than 100:1.