A towel is about the most massively usefull 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 vapors; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a minicraft down the slow heavy River Moth; wet it for use in hand-to-hand combat, wrap it around your head to ward off noxious fumes or avoid the gaze of the Ravenous Bugblatter Beast of Traal (a mind-bogglingly stupid animal, as it assumes that if you can’t see it, it can’t see you- daft as a brush, but very, ver ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems clean enough.
More importantly, a towel has immense psychological value. For some reason, if a strag (read : non hitchhiker) discovers that a hitchhiker has a towel with him, he will automaticly assume that he is also in posession of a toothbrush, washcloth, soap, tin of biscuits, flask,compass, map, ball of string,gnat spray,wet-weather gear, space suit etc etc. Furthermore, the strag will then happily lend the hitchhiker any of these or a dozen other items that the hitchhiker might accedentally have ” lost “. What the strag will think is that any man who can hitch the length and breadth of the Galaxy, rough it, slum it, struggle against terrible odds, win through and STILL know where his towel is, is clearly a man to be reckoned with.
Hence a phrase that has passed into hitchhiking slang, as in “Hey, you sass that hoopy Ford Prefect?… Lees gerust door