airtas

I bought an oscillating sprinkler to water my grass and noticed it was not going far at all. I turned up the main water valve and I tripled the output.

My question is why was only my outside water pressure affected and should the main valve be turned all the way?

Redwood

The main valve was restricting the flow of water to a degree where flow was reduced.

The valve should always be on all the way...

You may have a situation where you have high pressure (above 80 psi) and someone thought that partially closing the valve would reduce pressure.

This is wrong because as soon as the flow is reduced or shut off the pressure immediately will be too high again...

A pressure reducing valve would be needed in an application such as this and you will still get good flows.