The thin pieces of more fragile stones can be glued to a backing, making the thin slab much more stable and resistant to breakage. Sometimes, the thin pieces are also protected by adding a crystal-clear quartz or glass topper. Once these assembled pieces secured in a bezel, it can be impossible to tell they are not originally cut thicker. Assembling cabochons in this manner will make them more durable as well as affordable.
Sometimes a design calls for a cabochon with more visual "heft". Earrings could become too heavy if thicker stones are used. Bracelets may become too bulky and turn on the wrist, especially if the main stone is on the larger side as well as thick. When cost, size, and weight are a concern, raising a thinner stone with lightweight plastic shims can be the answer.
If a flat back cabochon is thin and more stone presence is desired in the piece being made, a wider bezel to cage the stone is used. The problem then is how to raise the stone. Steer clear of any products that can degrade over time such as carboard, thick paper, or sawdust. Yes, these will cushion the stone but if moisture gets to them, they can decompose over time, start to produce unpleasant odors, and no longer cushion the stone or hold it in the original raised position.
Thin plastic inserts, approximately .5mm thick, saved from sliced sandwich meat packages have made wonderful templates/stencils for a variety of often used shapes in my designs. They are easy to cut with a craft knife. I store mine in a three-ring binder in photo sleeves for large photos.