Well, because those of us who are diehard Tolkien fans know that he modeled the Shire on the English Midlands, while adjoining areas -- especially Bree and its surroundings -- were sort of like the lands of blended culture around Offa's Dike: the Englishness blends with Welshness. Bree comes from the Welsh word for "hill." Coombe is in a valley, which in Welsh is cwm. And so on.
But I didn't realize that Sarn Ford -- the main river crossing on the south border of the Shire -- was of the same sort. A ford enhanced by the building of a causeway. One word is Welsh, one English, right there on the border -- the interface -- between the two areas.