Hi Paolo,
My guess would be that it is a more realistic musical representation of a crescendo through a series of repeated notes. Similar to how they have crescendo and diminuendo patches on sustained pitches. Yes, you could achieve this by riding the cc velocity crossfader up and down, but it comes off more realistic when the players actually record the crescendo and diminuendo. To get the effect of the perf-rep-dyn patch, you could also ride your cc up or down while playing just the perf-rep patch, but again, I think pre-recording the succession of changing volumes is more convincing in the finished product. That is the mark of an excellent library - the multitude of articulations recorded in a variety of ways.