But the data itself is not new software. It is just data. 
  Functionality should not change because of new data.

Well, ideally, yes. But software breaks all the time when you give it data the programmer didn't expect, which happens especially often with time.

DST, for example. What if a state stops observing DST? Arizona observed DST in 1967, but hasn't since then. A couple other states are considering abolishing it, too. You can easily imagine software doing something dumb when it can't find its DST entry in tzdata.