Douglas Adams tells us that a hitchhiker's towel is the most useful article one can carry on any inter-galactic jaunt. It provides shelter from the sun (and the Ravenous Bugblatter Beast of Traal (sp??)), something to dry off with, and nurishment if you happen to have a so-equipped towel, as did Ford Prefect

More generally, this term was phrased as "He's a hoopy frood who really knows where is towel is", in reference to Ford Prefect, in The Hitchhiker's Guide to the Galaxy as being hitchhiker slang used to denote someone who had their very existence straightened out, right down to the minute detail of knowing where his towel was. So naturally such a person who has his towel with him surely (it is to be assumed, correctly or not) that he must have the rest of his necessities, such as toothbrush etc. also, and is therefore fully prepared for anything that may come.