The cans and lids provide protection before use and more importantly after they are shot. The value of the film and magazine are much higher after shooting than before. The cans and lids protect the valuable contents against light leak, moisture, dirt in the velvet, and physical damage. If your images are valuable put the mag back in the can/lid after shooting. If you don't care just toss it on the floor of your car.
Film can and lid plastic is recycled by labs.
Here is a little film can story. David Burnnet from Contact Press was accompanied by my boss on David's shoot in the Tokyo Fish Market on a cold day. David was reloading. He tried to pop off the lid with his thumb but it wouldn't go. He threw the can containing unexposed film in the fish muck and pulled out another. My boss asked him why he tossed a perfectly good roll of film. He said that he didn't have time to mess with a "bad" film can.
I was assigned to understand why the top was difficult to open and fix the problem. We changed the lid design and the plastic blend so the protection was still good and they could be openned in the cold. A small but important change. It was also expensive since new molds were required. For better protection Kodak Professional films had black cans.
To see how film is made: www. makingKODAKfilm.com