A tawa looks like a plain flat pan, and American cooks often substitute a nonstick skillet for one. The two are built around different assumptions about heat, coating and duration.
The surface is meant to be bare
A roti cooks against dry metal with no fat between it and the pan. Direct contact is what produces the brown spots and the dry surface that lets the bread puff.
Nonstick coatings interpose a layer that conducts differently and resists the sticking that browning actually depends on. Breads made on them come out paler and softer.
Seasoned cast iron or plain carbon steel builds a polymerized layer that releases food without blocking contact. That is the surface tawa cooking assumes.
Mass buys temperature stability
A raw disc of dough hitting the pan pulls heat out of it fast. A light pan drops in temperature immediately and the next bread cooks in different conditions from the first.
A heavy tawa stores enough heat that one bread barely moves the surface temperature. Cooking a stack of twenty back to back becomes consistent rather than progressively worse.
The tradeoff is responsiveness. A heavy tawa is slow to heat and slow to cool, so it is set to a working temperature and left there rather than adjusted constantly.
Prolonged dry heat is the operating condition
Most nonstick pans are rated for moderate temperatures and degrade under long dry heating. A tawa spends its working life empty and hot between breads.
That duty cycle is exactly what nonstick manufacturers warn against. Using a coated pan for regular roti cooking shortens its life considerably.
Shape encodes the intended bread
Flat tawas suit roti and paratha, where the bread lies fully in contact. Concave versions are used for dosa, where batter is spread outward from the center.
Convex tawas, curved upward, are used for very thin breads that would tear if handled flat. The bread drapes over the dome and cooks from the peak outward.
Choosing one for an American stove
Cast iron tawas are widely sold and work on gas, electric coil and induction, though a large one on a small burner will have a cold rim. Matching diameter to burner matters.
Uncoated tawas need seasoning and dry storage, since bare iron rusts. A well-used tawa darkens over time, which is the seasoning layer building rather than damage.