On a new construction build down the street, I am interested to know what this is and what it does? We are in California on public water if it makes a difference. 
This black thing on the main copper water line before entering house
copperwater
copperwater
On a new construction build down the street, I am interested to know what this is and what it does? We are in California on public water if it makes a difference.
Best Answer
A placeholder for the water meter [or a secondary water meter to differentiate internal (& sewer) water from external (just water) uses] is one possibility.
Or for some other (not presently installed) device that mounts with unions, such as a Pressure Reducing Valve (PRV.)