Years ago, "blue collar" workers took lunch boxes with them - so did most school kids.
White or pink collar workers didn't want to be grouped with those, so they took their lunch in paper bags.
Now we use the phrase "brown bag" when we mean "take your lunch", no matter what it is carried in.
It's a status thing, although probably few remember that.
That is ringing bells now with me years ago executives took bags out of brief cases at lunch time.
Nowadays there is not the stigma about it and they quite happily put the lunch box into the fridge at work in fact it is amazing for a place that only employs about 25 people we have 3 fridges, 2 toasters and a microwave pluse and several storage cupboards so people can bring stuff in and keep it fresh.