@Symbiont10000 already exappained the basic- it was't buying ready to eat food, but bringing food prepared- spiced, moistened etc- at home, just to cook it. Bread was baked nightly and large ovens in bakeries kept heat for hours afterwards, allowing to cook multiple meals at once before the dinnertime. It was a paid service but during festive it could be done for free as an act of charity.

Why this service was needed? Because clients using it probably didn't own proper home appliances to prepare roasted bird, meat or ham.

Urban poor dwellings were notorious for lack of any amenities- most of the flats/rooms were rented unfurnished and is tiled stove wasn't mentioned one knew that there weren't any heating system present.

So the stove the clients had was usually a simple movable wood and coal burning stove-looking like metal table with surface that could be used as stovetop to cook. The fancier ones had a small oven or roast, but the most basic ones lacked it.

Even if one had oven then you should have some cookware- back then tin, cast iron and earthpots were in use, and they were expensive to the point that single cast iron braising pot was a good wedding present for working class family and a possession worth mentioning in will.

But all of this was felleing- most of the working class lived in constant fear of poverty. One sickness of breadwinner could mean ruin and selling possessions on large second hand market. Reports of charity institutions paint sad picture of people who sold even beds and pots to keep themselves afloat.

Even if one had all amenities there was one more thing- fuel.

The most common type of stove was known in Poland by charming name od "koza" (goat)* because could be "fed" by anything flammable- wood, coal, paper, cloth scraps, garbage. The fuel was a huge position in budget, to the point that small kids following coal and wood carts or walking around railroad to get any scraps were a common sight.

To cook something bigger you need a good quality fuel that would keep steady, easy to control temperature for hours. When you saved for months or paid your bird on installments** so you can have once or twice a year fest you just can't risk. Not to mention that too large fire could be a fire hazard.

So- all things considered- I don't have exact sums, the ad shown in film could mean savings for working class family.

What to read: I used mainly "Życie Prywatne Polaków w XIX wieku" but if you are interested in background of reality described in great XIX century novels you can take "What Jane Austen Ate and Charles Dickens Knew: From Fox Hunting to Whist-the Facts of Daily Life in Nineteenth-Century England" by Daniel Pool

*https://i.pinimg.com/564x/b6/96/3d/b6963da5e98fa9cedf25f03d4d663c2b.jpg typical xix century stove- now I'm not so sure that's XIX century but construction was similar. It's one of the higher quality, the cheapest ones made from reforged metal scraps rarely survived. ** That was a thing- you either paid a butcher or paid someone to raise and kill animal for you. Used as a plot point in "The Adventure of the Blue Carbuncle" in Sherlock Holmes series