A towel, it says, is about the most massively useful thing an interstellar hitchhiker can have.

A towel has immense psychological value. For some reason, if a strag (strag: non-hitch hiker) discovers that a hitch hiker has his towel with him, he will automatically assume that he is also in possession of a toothbrush, face flannel, soap, tin of biscuits, flask, compass, map, ball of string, gnat spray, wet weather gear, space suit etc., etc.

https://www.goodreads.com/quotes/964943-a-towel-it-says-is-about-the-most-massively-useful