It is more a marketing thing, along with an added cost cutting. The marketing of "LED third brake light" was much more interesting to the consumer than "third brake light", as there were many vehicles with these, and many used incandescent lamps ( thanking you GM for this junk) for this.
A benefit of using LED lamps ( and when the manufacturer specced the lamps they also spec a brightness bin as well, so all the vehicles in a series will have a similar light output during manufacture, as this is part of the QC on parts, just that as led tech improved they moved down from best in batch, to mid batch, then finally lowest in batch) is that the current draw is lower. This means both a cooler running housing, so you can use a regular blend as used for the rest of the interior, instead of needing a high temperature plastic compound to prevent warpage. With this you also have lower current draw, under 2A for any light bar ( and some are really long and bright as well) as opposed to using up to 100W of incandescent light in a housing, which needs both heavier wiring in the car, a brake switch rated for this massive extra current draw, higher current fuses and supply wires and in many cases a BCU firmware change to handle reporting lamp failure. LED lamps thin wires, no heat sourse and no extra load over switch rating, plus BCU will still provide bulb out warning if the model has it.