An 18% gray card is an excellent tool for metering and just about essential for setting a custom white balance (although a piece of pure white paper will do in a pinch). A gray card needs to be large enough to meter and photograph without blocking the ambient light hitting the gray card, yet small enough to always have with you.
A rigid cardboard gray card works really well but it is not handy to carry with you.
Smaller, plastic gray cards are much handier to take with you, but they are so small it is harder to fill the frame with the gray card if you want to create a custom white balance.
Enter the collapsible, fabric, 18% gray card. When it is open it measures 12×12 inches so it is much easier to meter and fill the frame with the gray card. You have more working area than with an 8×10 cardboard gray card.
When it is closed it fits in a 6 inch zippered pouch you can easily slip into your camera bag.
You can get my favorite collapsible gray card at this Amazon link.