Because people have a need to glue things to other things
Paper to PlasticIf it's OK for the glue to show, we recommend:
But if you don't want the glue to show 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.