Are those necessary? Do they change the functionality similar to having a trailing semi-colon versus leaving it off changes from a statement to an expression? Or is this just a preference to make it easier to add items onto the list later?
Just to add to the other comments in this thread with a possible rationale. Adding a trailing comma can ease refactoring. Rust doesn't (yet) have amazing IDE support and forgetting to add a comma when adding a new field can feel a bit like a papercut.