It's there to complete the schedule. And it is not "breaching the FTL", it is a part of the FTL. It is true that pilots would be most likely to exercise the option if it happens to be getting them home but that is not what its purpose is. The rule makers don't give a stuff about where crews spend the night, they just want to make the rules flexible enough so that unforeseen circumstances don't completely wreck the schedule. It basically allows a company to roster to the basic limits, knowing that there is some fat left if needed.