The action tells if the advertisement needs to be added, updated or removed from the site. This is only applicable for interfaces using incremental updates, as sites using bulk interfaces determine the action themselves by comparing the batch data with the previous one.
DV keeps track of the current state of the advertisement and thus will only send only one (successful) add and remove mutation during the lifetime of an advertisement.
It is very much possible the action is not part of the mutation payload itself, but instead part of the API call being used (for instance, if the site offers the API "addAdvertisement").