[Too]

You are assuming that your usage of isReady will be flawless every time. What if you call a function updateUser("username", isReady()) but it turned out you remembered the order of arguments wrongly. Then the type checker will tell you as you can't pass a bool to a string argument and vice versa. This type of error is super common in %-string formatting.

It won't help if all the arguments have the same type unfortunally but at least it catches something. Just like unit tests, they won't catch everything so the more checks we have and the earlier we run them the less risk there is something slips through to production.