Why different types of materials are used has to do with the application and soil conditions and water quality and working pressures involved and of course how CHEAP the builder and or home owner want to cut corners.
Ever hear of “Jack” This moron Jack wanted CHEAP in not only materials BUT always looked for CHEAP/FREE advice and that is why when someone builds a crap house they say “This is the house Jack Built”
For example today it is muchfind skilled craftmen that work cheap so builders look for materials that any stumble bum can install like DO IT YOURSELF PLASTIC where no skills are required.
The same with gas and water distribution systems.
Look at the crap that the home owners are falling for with the “under slab heating applications” out of sight out of mind TIL it fails TA DUM.
Several materials can be used for various applications like copper can be used for heating, sprinkler, waste and vent and water supply BUT then comes the wall thickness like K L M and TP ACR and DWV etc.
Confusing huh? If you E mail me ILL be glad to go over some of the applications where we use schedule 10 20 40 80 120 black steel like fire supression systems.
Have a great one.