Useful for decorating.
If you're painting the walls with a roller you can line the paint tray with a carrier bag. When you've finished, remove the bag by carefully turning it inside-out, so that the left-over paint is inside it. Snip the corner off, and squirt it (a bit like an icing bag) back into the paint tin. Less waste, much less cleaning.
Whatever painting jobs you don't get finished and have to carry on the next day, you can make it quicker and easier by wrapping paint brushes and rollers in a plastic bag. That way they keep fresh and you don't need to wash them. Same goes for wallpaper paste brushes.
Also for wallpapering - line the bucket with a carrier bag (make sure there are no holes in it) before making the wallpaper paste. Again, when you've finished you can 'pipe' the excess paste into a jar for any touch-up jobs later on, and the bucket will be clean and not need washing. Or, if you don't finish the job on the first day, you can tie a knot in the top of the bag, or just fold it over to stop the paste drying out overnight.
I did wallpapering recently and searched high and low for a carrier bag, only to find I didn't have one that wasn't been used for something else. I'm really getting good at refusing them in the shops.
So although frustrated, I did feel rather smug.