"A towel, it says, is about the most massively useful thing an interstellar hitchhiker can have. Partly it has great practical value - you can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a mini raft down the slow heavy river Moth; wet it for use in hand-to- hand-combat; wrap it round your head to ward off noxious fumes or to avoid the gaze of the Ravenous Bugblatter Beast of Traal (a mindboggingly stupid animal, it assumes that if you can't see it, it can't see you - daft as a bush, but very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough."
quotation from The Hitch Hikers Guide to the Galaxy by Douglas Adams www.douglasadams.com
Unsurprisingly, the same holds true for getting around Japan. A towel is one of the more useful things you can have in your possession at most times. It is useful for, among other things, fashioning an impromptu gaijin twist-headband or turban, placing in your lap as a substitute for the traditional western napkin (Japanese napkins are tiny rectangles of nonabsorbant tissue paper measuring perhaps 6"x8" unfolded), mopping the sweat off your face after a few hours running around one city or another in the hot muggy pseudo-summer months, sheltering from the rain when you accidentally leave your umbrella at the office/apartment, or simply drying your hands (the bathrooms near the ETC office are strictly bring-your-own-towel). Traditional Japanese towels are not incredibly absorbant by western standards, so bring your own bath and beach towels if you want them. A small Japanese hand-towel, however, will suffice for day-to-day keeping-with-you, and should run no more than $3 (300 Yen) at a local convenience or souvenir store.