For a start, you could use them for carrying your stuff into the festival. They are bags. Nowhere near as sturdy as a rucksack, but still bags. And if it's raining on the way in, you could cover your rucksack with them to keep your things dry.
This is fundamentally a highly flexible, waterproof sheet. Punch three holes in it and it's a raincoat. Wrap a couple around your feet and they are rain-boots. Place one on the floor and it's a dry patch upon which to sit.
Place one at the entrance to your tent (particularly during a Mudfest) and it's somewhere to take off your boots. Drape your belongings in them to keep the mud out of your clean things. Put your muddy things inside them. Mud damage limitation for the win.
My mind flits back to that Glastonbury when a friend arrived with a 'bargain'. It was a very, very cheap tent. Our dubious expressions were vindicated when she erected it. It was a children's play tent, ideal for the living room, not so wonderful for the rigors of a week at the Glastonbury Festival.
Nevertheless, all was well until the third day, when it rained. At the first spot, our heads turned in unison, like horrified merecats, towards that flimsy structure. You've never seen a bunch of hippies move so fast. Bin bags came from every tent and we had it covered within the minute.
It was very noisy, with thin, plastic sheets rippling in the wind, but it kept her play tent dry for the entire festival.
So you can wear it, sleep in it, create a tent from it, waterproof your dwelling with it. Oh! And bin bags can hold your trash too. Most important thing to take beside your ticket.