Function of Reversible Contact in Lower Size Pump Motor Circuit

It depends on specifics. For some types of pumps, if the pump gets clogged, running it backwards might clear the blockage.

Reversible contact allows the pump to work in both the Northern Hemisphere and Southern Hemisphere without replacing the contacts when crossing the Equator. (See Coriolis Effect)

This is not to be confused with the multi-hertz contact that allows operation at both 50 and 60 hertz, but is not hemispheically sensitive.

Hope this helps.

I have seen in few projects -- for higher size motors this contacts are not used.. What is the reason behind?

I'm headed for the naughty step, I just know it.

Del's already on it. Find another one....

Hey, this step is getting crowded

Sounds plausible to me

In the pump casing of centrifugal pumps direction rotation of shaft/impeller is marked permanently. If we change direction of rotation in either northern or southern hemisphere

will the rate of flow change?.

Yes. Wrong direction = lower flow and less pressure capability.

May be for chosing the right direction of pump. without changing the phase sequence / terminals , to be user friendly & convinient for house people to use pump properly.

I can think of two reasons, one already expressed by Tornado and the other alluded to by Harry Potter:

1. Reversing of grinder or chopper pumps that become jammed
2. Maintaining proper rotation on 3 phase motors when powered by emergency generators. Most sewage lift station pump stations and many water booster stations are fitted with plug-in portable generator receptacle so that they can be powered in emergencies. In several cases I have worked on, they wanted them to be powered by any Em. Gen, but for whatever reason the Fire Dept. generators were not coordinated with the Sewer or Water Dept. Em. Gens., so in case the FD got there first, they would have to change rotation to make the pumps work.