Cultural significance
Towels have long been thought of as nothing more than utilitarian objects that everybody has, but about which nobody really thinks twice. This changed when Douglas Adams' The Hitchhikers Guide to the Galaxy attained cult status in the 1980s. He described his characters travelling around his universe, often as hitch-hikers, finding that towels were the most "massively useful" objects they could carry. The fictitious time/space traveller and Guide Researcher Ford Prefect uses the idiom "a guy who always knows where his towel is" to mean someone generally alert and aware, someone who in 1960's Earth slang might have called "with it".
Fans of Adams' books have seized on this idea, and towels are now considered a symbol of one's devotion to the Hitchhiker books, radio series, TV series, website, etc.