If I recall correctly, FAF is a fix. A two dimension fix on the approach path, where the final segment begins. The FAP is the point along that path where you capture the glide slope.
I don't know why they make a difference. Maybe because different true altitude depending on temperature, or ILS beam bending or something like that? Maybe because of the nature of the approaches, FAF in NPA, FAP in PA?. It is not mandatory to start descending at the FAF. It is mandatory not to descend before it, which is different. But reaching the FAP, at the published altitude, you have to intercept and follow the G/S.