There is no Open Source software that is not free, if we are using the OSI definition of Open Source. FOSS just adds a certain emphasis for reasons of the historic tension between the FSF/GNU "free software" advocates and the "open source" advocates. FOSS doesn't mean that OSS is not free without the F.
There are a few twits out there who believe (or at least troll) that proprietary software whose source code is available is "open source". So FOSS provides the minor and largely unnecessary benefit of making it clear that it's not that.
There are a few twits out there who believe (or at least troll) that proprietary software whose source code is available is "open source". So FOSS provides the minor and largely unnecessary benefit of making it clear that it's not that.