The plastic canister is mainly extra protection for the film cassette during storage and handling.

A 35mm cassette is not perfectly sealed against the outside world. The film has to pass through the cassette’s felt light trap, so dust, small amounts of moisture, and some stray light can potentially get in. The outer canister adds another layer of protection against those hazards.

It also helps protect the cassette itself from being knocked open or damaged. Film cassettes—especially reusable ones—can come apart more easily than you might expect, while the plastic container keeps everything together and safer in transport.

So the canister isn’t redundant: it’s a simple protective package that helps keep the film clean, dry, dark, and physically secure until use.