It's a combination of wanting it at random times, and it being easier to just leave a few foil-wrapped teabags in my backpack (and in my winter coat pockets), because they are small, light-weight, and don't spoil quickly.

This has the advantage that "I'd just like a cup of tea" isn't an imposition even for people who never drink the stuff and may not have any around; it's a rare household that can't boil water, maybe using a saucepan or the microwave.