According to NFPA 20, section 2-9.5:
"A listed outside screw and yoke (OS&Y) gate valve shall be installed in the suction pipe. No valve other than a listed OS&Y valve shall be installed in the suction pipe within 50 ft (16 m) of the pump suction flange."

I was wondering why there is not a similar requirement for discharge pipes and OS&Y valves in section 2-10, as a manner to instantly know if such pipe is open or closed.

Maybe somebody can clarify me the purpose of this part of the code, and if it?s a good idea to install OS&Y valves in discharge pipes.

It is my understanding that you do not want anything causing non laminar flow.  The other commonly used valve is a butterfly valve which has the vane in the waterway at all times.  An OS&Y gate valve does not have anything in the waterway.

You are required to have an indicating valve.  A butterfly valve is an indicating valve.  You can tell if it is open or closed as soon as you look at it.

There have being incidents on which a butterfly valve was installed and when th pump activated the flapper on the butterfly did not fully open causing the piping and valve to cavitate. This will not happend with the OS&Y valve.
This requirement has being in the code for quite some time now. I have seen incidents where the spacing in the fire pump room did not allowed for an OS&Y screw yolk type valve and a butterfly was installed. Not a good practice.
You also have the stainer distance requirement 10 times pipe diameter away from suction.

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