A towel is about the most massively useful thing a con-goer can have. Partly it has great practical value. You can wrap it around you for warmth as you walk in front of the Operations Suite filled with the coldness of the Yojimbo's hearts ; you can lie on it in the park as you watch the silly glomp circle; you can sleep under it in the hallways because your roommates kicked you out; wet it for use in hand-to-hand-combat (outside the con space of course); wrap it round your head to ward off noxious fumes of the gaming room; 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.
More importantly, a towel has immense psychological value. For some reason, if a normie (normie: non-con-goer) discovers that a con-goer has his towel with him, he will automatically assume that he is also in possession of a toothbrush, washcloth, soap, box of Pocky, Ramune, manga, deodorant, vender's hall map map, narutard spray, swim suit, laptop computer etc., etc. Furthermore, the normie will then happily lend the con-goer any of these or a dozen other items that the con-goer might accidentally have "lost". What the normie will think is that any man who can attend a con, rough it, slum it, struggle against terrible odds, win through, and still knows where his towel is, is clearly a man to be reckoned with.