Call it courtesy or CYA ( I may be the one to make the changes) I like to put a union in occasionally. Some plumbers don't like unions because they have to be exactly in line and exact fit to keep them from leaking; no unions, no problems.
You may cut the line where ever you need to make a change, take the pipe from each side of the cut and have the local hardware store to thread them and cut one side in two pieces, threaded and include a union. This allows you to insert what ever you need without having to unscrew the whole house of pipe. Caution:be sure you cut out some pipe to allow for the extra fittings you are installing.