Because people have a need to glue things to other things
Metal to PlasticWe recommend:
Clamping can make the difference between success and failure here.
Whenever you are gluing metal it's a good idea to clean it first with steel wool or sandpaper. (Rust never sleeps.)
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.