Maybe the wine at the curry house was shit and they knew it so they brought their own.

Sometimes you can bring a cheap bottle of wine and even with the corkage fee it ends up being less than the cheapest bottle of wine at the restaurant.

It's just a really easy thing for restaurants to do. Even if it's not listed many will do this.

I have only ever encountered it in the context of wine though. I wonder what would have if I brought a bottle of whiskey.

If you were a wino and that was your favorite restaurant, why wouldn't you?

Does that curry house have an alcohol licence? Because some don't (either for religious reasons or cost, I'm not sure), but they're still providing glasses that will need to be washed.

Generally this is done for cost reasons by the customer. Wine in restaurants is expensive, corkage doesn't make up the difference, so people looking to save some money will bring their own, pay the fee and still come out on top. Fancy restaurants with sommeliers don't advertise this (idk if that means they don't do it though)