The indo-western gown is often described as a halfway house, which undersells it. It is not Indian wear made more Western or Western wear made more Indian. It is a construction that solves a problem neither solves cleanly: a formal evening in a venue built for Western formalwear.
The reception is the obvious case. It is usually in a hotel or function centre, usually seated at tables, usually involves a long evening of being photographed standing up and then sitting down again. A floor-length gown with Indian surface work reads correctly in that room in a way that a full lehenga sometimes does not, and it handles the sitting-down part better than a saree.
The pre-draped saree gown is the most interesting version of the idea. It keeps the pallu and the visual language of a saree but builds the whole thing as a single garment. You get the line without the maintenance.
The fish-cut and corset constructions go further the other way — the silhouette is Western, the work on it is not.
