This is also known as loose polyester fibre fill. It is used for stuffing, furniture back cushions, sleeping pillows, throw pillows, stuffed animals, crafting, pet beds, round bolsters, etc. It is naturally hypo-allergenic and has anti-bacterial properties.
If you have an existing cushion that has lost its loft and it is no longer supportive, then you can use this to bring it back to life. Just weigh the cushion and this will tell you how much fill you need to replace the old fill. An average sofa back cushion weighs approximately 5 lbs.
If you want the best of both worlds, you can go with a thinner piece of foam and then wrap it a couple of times with some thicker 2” or 3” Dacron fibre, see Fibre Wrap.