I am replacing the shower rough valve for my tub/shower and the inlet lines were tee’d with a vertical section of pipe and capped off. Why was this done and do I need to do the same thing when I go to install my new one? I’m using PEX for the new valve if that makes a difference.
That's an old-school water-hammer arrestor. The air bubble trapped in the extension serves to limit water hammer, at least until it all dissolves.
They work if you drain the pipes (to restore the bubbles) when they stop working, and have no mechanical parts to fail.
Modern hammer arrestors try to prevent the dissolving problem by using a piston to separate the water and air, but then have mechanical parts that can fail.
You can use hammer arrestors of either sort, or choose not to, though you may find yourself retrofitting them if water-hammer is a problem in your house without them.
