What is the purpose of this blue handled valve?

[Longboat58]

I am wondering what the purpose of the blue handled valve is? I'm going to be replumbing and greatly simplifying the section leading from the filter to the black valve on the right as the grey device (not used and not to be replaced) is cracked and leaking. I don't know if I need the blue handled valve or not. In 18 years I have never touched it. Thank you.

 

[1poolman1]

I am wondering what the purpose of the blue handled valve is? I'm going to be replumbing and greatly simplifying the section leading from the filter to the black valve on the right as the grey device (not used and not to be replaced) is cracked and leaking. I don't know if I need the blue handled valve or not. In 18 years I have never touched it. Thank you.
View attachment 474131

Appears to force water through the Nature2 . Would help to see a view from the front, facing the building.

 

[Longboat58]

Appears to force water through the Nature2 . Would help to see a view from the front, facing the building.

I added a front picture. But how would it "force" water through the Nature2 if the valve is in the open position? Thanks

 

[1poolman1]

I added a front picture. But how would it "force" water through the Nature2 if the valve is in the open position? Thanks

When it is closed all the water would have to go through the Nature2. You would do well to remove both that (the replacement cartridge is over $100 and you don't need those minerals in properly treated water), the tablet feeder (get a floating tab feeder if you're going to use tablets), and the unused injector venturi that was probably used for an ozonator. Wouldn't be real hard to plumb out of the filter into the return valve.