I use KNO3 as part of my automated dosing schedule, I used it heavily when I had an SPS dominated reef and continued when I restocked to a more soft coral dominated display.
It's possible to use nitrate dosing to lower phosphates in a reef, which is a prevalent issue especially when feeding heavily - most prepared foods give a lot more phosphate rich waste than nitrogenous, and bacteria need higher proportions of nitrate than phosphate to metabolise this.
I've actually combined my carbon dosing with kno3 and dose around 4ppm of potassium nitrate a day. I've noticed when I don't dose this mixture, the algae growth on my glass slows and I end up with cyano prevalent if it isn't redosed for a few days. I see similar when my skimmer stops working so I believe it's a result of reduced bacterial metabolic action.