There's no reason value objects need to be mutable. In fact, they shouldn't be mutable. They're just data. Why would you need to mutate them?

Well, in the strictest since, I suppose a "value object" is technically one that is immutable by definition. So, to your point, I should clarify that here I'm instead speaking more loosely of an object that is chiefly comprised of data elements vs. behaviors.

So, a common use case might be loading data from a DB, changing several fields, then writing updates back to the DB.