Advantage of Flowing More Water in a Flow Test?

I've come across this question - why do we need to flow more water - from two angles: as a total rookie, and later on as someone needing to really understand a water supply.

As a newbie - I was intimidated by a few things; first, that someone would call the police on me because I didn't look like I knew what I was doing. Second, that I didn't want to destroy any landscaping. And third, I definitely didn't want to be breaking any hydrants. Those three factors made me want to keep my flow tests as calm and low-flow as possible. 

However, as I was told at the time, that's not advantageous when we're trying to determine the quality of an existing water supply.

Just a year ago, I was working on a project with a marginal water supply, where the water tower and the pumps feeding it were controlled by the project owner. The tower was in some disrepair (not known to us at the time), and we were trying to figure out why we were getting such different results from what should have been a fairly consistent supply. 

It was on this project where we really needed to understand the strength of the supply that was well beyond just 300, 400, or 500 gpm into the system. But why?

Why does it matter if we flow 500 gpm or 1,000 gpm when doing a flow test?

One perspective - and one answer to this - is confidence in the data. We gain more confidence in our test results with the greater amount of water we flow. Here's a video we put together that explains this perspective a little better: 

Hope you have a great week!