I installed and use my FlowVis primarily to determine the flow through my solar panels, which expect a certain amount of flow to optimize the heat exchange. For me, it was a valid expense. I think there is a way to calculate flow, but because I needed a check valve anyway, for the solar, the difference in cost between the check and the FlowVis made sense for my application. Just in time alone (time I didn't have to spend calculating flow otherwise, plus the bigger chunk of time: learning how to do so!). Calculating flow would have been a guesstimate at best, with the Vis I'm confident I know exactly what it is (within its margin of error, of course).
I have since used it to optimize flow through my SWG, and for my suction-side vac. And I monitor flow for general plumbing "health," and as a backup to my filter's pressure gauge.
Other than the solar use, I'd say a flow meter is a luxury, not at all a necessity.