Ok. Think of the following situation. There is a GPX track and you want to delete some points. Doing this with JOSM you can open the track, convert the new GPX layer to a data layer and then mark point or regions and delete them. At the end you can convert the data layer back to GPX. For small tracks this is no problem. But while editing tracks with thousands of points JOSM is getting extremly slow. Try it yourself.
With the EditGpx plugin you can delete single points or parts of huge tracks very easy and fast.