Because people have a need to glue things to other things
Plastic to PlasticIf you are gluing PVC plumbing pipes, we recommend:
If you are gluing something else and your joint needs to be flexible use:
There are so many kinds of plastic its hard to give advice here that applies to them all. If possible try a small test in an area that doesn't show.
Some plastics have a smooth surface finish that can be sanded off with a 120 grit sand paper, for better adhering properties.