Since the line can be hard to see sometimes, it helps allot to wiggle it a little bit, so the eyes can easely find it on the screen. Detecting motion is somethings that our eyes and brains are good at.
This is something that I think would be helpful allso while scrolling, but more importantly:
When checking the forecast for two different times I would, while I keep my finger down on the screen, scroll, wiggle, swipe to second time, wiggle, swipe back to first time, scroll to see the other data sourses, wiggle, swipe to second time, wiggle, swipe back to first time.
So being able to seamlessly, without lifting finger from screen to put it back down, perform swipe and scroll, is for this reason important.
To be forced to do a release with finger off screen between every action would potentially cause much frustration, (as is does in the other app i described).