Lots of things, but I will focus on one point that was not mentioned by the other answers :
They make the character of Anakin Skywalker aka Darth Vader obsolete, at least his role in the saga.
In Episode I, The Phantom Menace, young Anakin is presented as a Force anomaly. He has no father, only a mother, and is literally created by the Force itself.
It won’t take long for the Jedi, at least some of them, to consider that he is the chosen one of a prophecy, supposed to restore balance in the force.
Except that, after several adventures (which I will spare you here, I invite you to read what I have already written on the subject in one of my previous answers: What do you really like about Star Wars? ) Anakin switches to the dark side of the force, and becomes the emblematic Darth Vader.
He will thus eliminate a very large number of force users, during the attack on the temple but also in the events taking place in the 20 years between episode 3 and episode 4.
As the venerable Master Yoda says in Episode III, the prophecy may have been misinterpreted. This is probably the case since in Episode VI, the return of the Jedi Anakin in the form of Darth Vader sacrifices his life to save his son Luke, by killing the last Sith Lord: Darth Sidious. He will lose his life there.
So every representative of the force, from the light side, as well as the dark side, was eliminated by Anakin/Darth Vader, which in itself is an interpretation of the return to balance of the force.
Indeed, the notion of balance has since been evoked in several other Star Wars works as a perfect balance between the dark and light side, and not just the presence of the light side.
Thus making Anakin the true chosen one of the prophecy , and giving this iconic antagonist a whole dimension of tragic hero. This is the key element of the saga.
All this made sense before…
…The return of Dark Sidious in the very bad episode 9. Anakin is therefore officially no longer the chosen one… he was useless, like all the elements that George Lucas had put in place all these years.
Thank you for reading, please share your opinion in the comments!