Indian lemon water usually contains salt as well as sugar, which surprises anyone used to American lemonade. The salt is doing two separate jobs, one on the tongue and one after swallowing.
Sourness is suppressed rather than removed
Lemon juice is dominated by citric acid, and the sour sensation tracks acidity closely. No amount of sugar lowers the acidity of the drink; sugar only competes for attention on the palate.
Sweetness and sourness suppress each other perceptually. A glass with more sugar reads as less sour without any change in its actual acid content, which is how American lemonade is balanced.
Salt suppresses sourness by a different route, and it does so at concentrations far below what registers as salty. A small pinch shifts the balance without making the drink taste of salt.
Two suppressors beat one
Using sugar alone means adding a great deal of it to tame a sharply sour juice. The result is heavy, and the sweetness lingers rather than clearing between sips.
Adding salt lets the sugar level stay lower while the drink still reads as balanced. That is why nimbu pani tastes lighter and more thirst-quenching than a sweeter lemonade of the same acidity.
Salt earns its place in hot weather
Sweat carries sodium out of the body, and plain water replaces the fluid but not the sodium. Drinks that include salt address both at once, which is why the salted version dominates in Indian summers.
Sugar in the glass is not only flavor either. Sodium and glucose are absorbed together in the small intestine, so a drink containing both moves fluid across faster than water alone.
This is the same reasoning behind commercial rehydration formulas, though a home glass is not a measured medical preparation and should not be treated as one.
Black salt is a separate ingredient
Kala namak is not simply salt with a different grain. It carries sulfur compounds that give it a distinctly savory, faintly eggy aroma, and it changes the character of the drink noticeably.
US supermarkets rarely stock it; Indian grocers and online sellers do. Regular table salt is a workable substitute for the suppression effect but produces a plainer, less savory glass.
Order of assembly matters more than it looks
Sugar dissolves slowly in cold water, and undissolved crystals at the bottom mean the top of the glass tastes sour. Dissolving sugar and salt in a little warm water first avoids that.
Lemon juice goes in last because prolonged contact between juice and ice dilutes it unevenly. Squeezing directly over the glass also keeps the aromatic oil from the peel in the drink.