Indian kitchens that own a powerful blender often still keep a stone wet grinder. The machine survives because grinding and blending do different things to a wet batter.
Crushing and cutting are different actions
A blender uses fast-spinning blades that cut and shear whatever passes them. Particles get smaller through repeated slicing, and the process is violent and quick.
A wet grinder rolls heavy stones over a stone base, crushing material between two surfaces. The action is compressive, closer to what a mortar does, and much slower.
Crushing ruptures rice and lentil particles along their natural structure rather than slicing them arbitrarily. The resulting batter is smooth without being pulverized into a paste.
Temperature is the practical divide
Blade speed generates significant friction heat. A long blending run warms batter noticeably, and that warmth affects the microbes that a fermented batter depends on.
Stone grinding runs slowly and the stone mass absorbs heat, so batter comes out close to room temperature. For preparations left to ferment overnight, the starting temperature matters.
Air is worked in, not knocked out
The rolling stone folds air into the batter continuously over twenty or thirty minutes. That incorporated air contributes to the lightness of steamed and griddled preparations.
A blender incorporates air too, but the shear that follows breaks the bubbles as fast as they form. Batter comes out denser even at the same apparent smoothness.
This is why volume increase is used as the sign of a finished grind. The batter should visibly grow in the drum rather than simply become uniform.
Water is added differently
Grinders take water in small additions across the run, and the batter stays thick enough for the stones to grip. Too much water and the stones slide without crushing.
Blenders need enough liquid to keep material circulating past the blades. That minimum is often more water than the recipe wants, and the batter ends up loose.
What this means in an American kitchen
Tabletop wet grinders are sold through Indian appliance retailers in the US, usually as compact two-stone models rather than the large versions used commercially.
For occasional cooking a good blender is workable if the batter is ground in short bursts with rest periods and as little water as it will tolerate. The result is serviceable rather than identical.