I'm not sure what they are used for, but after reading a thread about someone losing prime and no water was running through the pipes... I figured, hmmm, maybe that would let me 'see' if water is actually running through the pipes when I turn the pump on for whatever reason. Maybe you can feel vibrations and water flowing, but seeing the water go through might be a better idea?
Can you actually "see" water flowing if it's clean water? Hmmm. So, should I put one in next to the pump? Is that what it's used for? Again, I'd rather do it now, than cutting it in later after the install.
I just have a pump and swg. nothing else. I see pictures of peoples' piping and wonder how the heck do they know where all those pipes go!? lol Too many valves for me.