Most polymers are made from crude oil, which is a limited resource that cannot be replaced. Recycling polymers means the plastic is melted and made into new products, so less crude oil is needed as a starting material.
Recycling also keeps polymer out of landfill sites. Polymers are not biodegradable, so waste plastic stays in the ground for a very long time. Burning waste polymer gives off gases that can harm the environment, and recycling avoids this too.
There are drawbacks. Different polymers cannot be mixed when they are melted, so the waste must be sorted by type first. Sorting is slow and the cost is high. Collecting and transporting the waste also costs money and uses fuel, and some polymers weaken each time they are recycled.
To evaluate recycling, weigh the saving of crude oil and landfill space against the cost of collecting and sorting. It makes economic sense only when reprocessing costs less than making new polymer.