Everyone can be certain the way it’s written is the way it will always run. This code is also tagged with someone’s address. The owner of that address gets to decide what operations are open to the public and what only they can execute. They only get to make this decision at the time the code is written. Once written, it cannot be changed. Everyone will still be able to see the code and what it’s doing, but can only interact with it in the ways specified by the owner.
Let’s start with the original motivation to create a blockchain: money. Our current monetary system is based on records of how much money is out there and who has how much of it. We rely on our governments and banks to maintain these records. But a blockchain allows us to keep these records ourselves, since it guarantees that the record is the same for everyone.