Why wouldn't the husband just take lunch with him?

I'm taking a guess, but I think it's because, since it's made from scratch, his wife would have to get up very early to get it ready by the time he was ready to go to work. Also, I don't think it would have to be re-heated if it was delivered within an hour or two of it being made. But, what do I know? I subsist on frozen food.

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Indians usually like home-cooked meals and the dabbahwallas are a service that allow you to cook in a relaxed manner and also serve it hot. It's much easier that way since people who don't have a family can register with a restaurant or "cook from home" lady who will send you your food. You don't have to walk all the way to a restaurant or cook for yourself.

It's a Mumbai thing.

Mombai (formerly Bombay) is one of those cities where every second matters. Life is competitive, public transport is at maximum capacity but still on time. So no waiting in the morning for the box to be packed. Even a few minutes would mean missing the train which could mean a colleague would get the chance to impress the boss that day.

You won't see something like this in other Indian cities. People just take their lunches with them or buy lunch. Note that as the film shows not everyone does this even in Mumbai there are those who get by on fruits or more commonly just take the box with them. However since the system exists, and the wife isn't working in this case and can cook later in the morning and send the food over they use the system.