I use them as painting drop cloths!
You didn’t give information about the type of curtain liner you have and that would give better answers. I’ve used vinyl liners a few different ways in the past. I’ve used them as drop cloths when painting but keep in mind they are slippery and don’t absorb any paint. I’ve used one to cover the porch when redoing planters that are too heavy to carry. Use the liner as a collector for the old plants and dirt. When my husband has to do yard/pond repairs (when the ground is wet from a leak or rain) he uses a vinyl liner as a dry layer to lie on the ground so he doesn’t get wet.
I save them for messy outdoor projects. Painting, repotting plants, etc.
We put one under the high chairs at mealtimes, then carry out the mess out to the yard. Shake off messes and rinse with hose if needed. Keeps my floor so clean!!
I use one for hanging over my south facing window in the summer time. It is amazing how much cooler it stays compared to curtains. The key seems to be stopping or blocking the sun before it enters the house. If the shower curtain liner is on shower curtain rings you can easily open and close it as you please.