Why do some objects have a hammer icon and some don't?

In order for the object to function fully, it first needs to be completely built or restored. Depending on the object, this can happen in several stages. The hammer icon indicates that the object needs to be restored/built before it can be used further. If there is no hammer icon, it means that the object has already been built or restored, but that it can be upgraded by tapping the object and entering the window of the object.