This is not necessary here. Additionally we can use O_CLOEXEC to make sure the file descriptors are not leaked by accident.
It is how it really works. Think of as a handle value, you're promising not to assign that handle value to point somewhere else, not that you won't change the value it points to.
Why i want this const? Because when you see that you can be sure that the rest of the file there's no other fd = open() or similar, and thus you're sure that the rest of the operations in this function that use fd are over "path".