After countless loads of laundry, your washing machine may well no longer smell quite as fresh as it did when you first had it installed.
By nature, washing clothes creates a pretty damp environment, and can therefore result in a pretty hospitable environment for mould and bacteria to flourish.
Dark marks on the rubber seal of your washing machine may well indicate yours is ready for a bit of a scrub, but many don't know how to go about it or else fear causing accidental damage.