First it is important to understand that the tap is a valve which connects the property owners water main, to the city main. The tap serves as the on/off switch controlling water from the city main into your private water main.
In most cases when you are doing a standard upgrade of your water main it will include connecting to the existing tap connection. In a case like this the only reason to replace the tap would be if it was leaking or if it was no longer functioning. Unfortunately the condition of the existing tap remains unknown until the plumber excavates and exposes the tap.
It is common for a property owner to question how the plumber is not aware that the tap is not functioning prior to the estimate? It can certainly become frustrating when you find out after the fact that your tap must be replaced, unfortunately there is no way for anyone to know the exact condition of the tap until it is exposed. For this reason there may be a change order on a standard water main installation once the plumber becomes aware of any issue with the tap.
The three most common issues a property owner will experience with a defective tap connection are a leak at the nut, a leak at the screws where the tap is installed into the city water main and if the valve is in such condition that it simply no longer works.
Fixing a leaking tap usually ends up becoming a larger and more timely repair than one would expect. The plumber is now required to get the city involved to close the city water main before the old tap is extracted, and the new tap is installed. In some cases the city will use the existing hole in the city water main for the new tap which saves time and avoids any further damage on the city main. If the existing hole is not usable the plumber is required to put a stainless steel clamp over the city main and the DEP will drill a new tap connection into the main.
Once completed the plumber is required to connect the new water main and obtain an inspection by the NYC DEP.