Some time, Requirements are not clear, or development team gets a working old application and ask to treat that as requirement. Project manager does not make SRS or get singoff for that as assuming working app as SRS. This turns out as problem later on, end client will come up with some requirement which was hidden in working application, or some security requirement or say browser compatibility. From customer point of view these hidden features or security requirements were obvious and for successful project these should be implemented, but these things are ignored by development team unknowingly. These issues can be sorted out before starting project with SRS signoff; and all the stack holder in projects are aware off what will be the outcome of the project.