"Researchers attribute the blood-pressure lowering effects of beets to their high concentration of nitrates" I don't get why Nitrates in beets are a good thing

"Researchers attribute the blood-pressure lowering effects of beets to their high concentration of nitrates" I don't get why Nitrates in beets are a good thing while they seem to be bad in spinach, kale and brussel sprouts. The really "bad" stuff seem to be Nitrits that are converted from the Nitrites by cooking. That would go for all cooked vegetables, though.