At their worst, they're artless, pointless; form without function, retelling an old story without understanding what made earlier versions worth hearing, and without adding anything worthwhile. (Star Trek Into Darkness, I'm looking at you.)
At their best, though, sequels and reboots are a chance for a new generation to take an old story and say why it still matters, to point to certain aspects and say, "Hey, this is a thing of relevance to us."
And right now, Star Wars is there.
When The Force Awakens came out in 2015, the most common criticism was, "This is just a remake of A New Hope." The Empire was already beaten, critics said; why was the First Order a thing? Why rehash this same old fight, in this same old way, against a same old foe, just with a new generation of heroes?
But if there is anything that the two years between Episode VII and Episode VIII have taught us, it is this: Wars do not stay won on their own.
Ideology resurges. If you don't fight the Nazis in every generation, they get new clothes and come back, with allies in places they should not be.
Star Wars ended up being accidentally prescient. But it will not be the last popular art of this era to have to engage with that idea. It is the challenge of our culture, right now, and it will continue to emerge in all the stories that were started this year, and that will not see the light of day for months or years to come.
Maybe sometimes we have sequels for a reason.