It’s about separation of concerns. The courts use precedent to ensure fairness through uniform interpretation of the law. It’s the legislature’s job to update the law to reflect what’s best for a changing society: when the text of a law changes, the courts obviously need to take those changes into account.

Precedent isn’t really about outcomes, it’s about lines of reasoning. If the law says that something has to be purple, the courts may need to decide what standard to use for purpleness. If another case then comes along that requires something to be green, the courts will try to make their greenness test work the same as the earlier purpleness test as much as possible. That way, everyone can get an understanding about how the courts reason about color. If the legislature disagrees with the court, they can amend the law to specify more precisely which color they wanted.

There’s a little bit of extra subtlety in that the US court system is hierarchical, and precedent is only controlling if it came from a court in your part of the tree. Different circuits may apply different reasoning for the same issue, and that’s probably the most common kind of case for the Supreme Court to take: two competing standards are in use, and they need to decide which one the whole country should use.