Soundproofing a room is very different than acoustic treatment for optimizing sound quality WITHIN the room. Egg crates will be completely useless for soundproofing, which requires significant construction considerations.
But even for simple acoustic treatment, egg crates are of questionable efficacy. There is just not enough material in the thin egg crates, their pockets are not deep or random enough for good broadband diffusion, and the material itself isn't a great absorber compared to alternatives.
