Star Wars (aka A New Hope) serves as the perfect introduction to the saga. It tells us what the Force is, and what Jedi are.
When ANH isn't first, the exposition on Tatooine can seem redundant to anyone who's already seen the PT. Especially coming after the galaxy-spanning action in ROTS, ANH seems to take an unnecessary detour.It slows down the larger story.
With the exception of Palpatine's identity, all of the jokes, references, and reveals of the saga only work if you see the OT first.
As we can see with Darth Bizarro's sister, when you watch the PT first, Vader ceases to be the badass villain in the OT. Instead he becomes "emo Ani", the pitiable burn victim who was tricked into the turning to the Dark Side.