Sorry to be vague, the valve would be used to control the flow rate (gpm, cfs) out of pump. (My preference is to use a stepper motor on a pump, but that's expensive for the rates I would like to see.) I can think of a few possible applications:
1. When using a reverse chiller, to slow pump rate down through chiller to improve heat exchange, until outflow temperature of wort reaches desired point.
2. When recirculating, sparging, and draining mash tun, to prevent negative pressure under grain. And prevent a stuck mash.
3. To control how fast wort moves between tuns and fermenters.
4. To eliminate the need to stir the tun with a mechanical vane, just let the pumped water do it.
There may be some problems because the valve can create a Venturi effect on the fluid.