Well @Behemoth (using my best old man voice) you young whipper snapper, Treasury Editions were huge prestige comics that contained either reprints of classic stories or all brand new stories. They were the graphic novels in the age of bear skins and stone knives.
Look at the beauty of it! A giant $2.00 book in the days of the $0.25-0.35 comic. Art larger than life! Look at the detail of the Neal Adams art. Every tiny line in exquisite detail! What is the value? Priceless!
These were the days of the spinner rack. No comic book stores. If you missed an issue there was no place to buy a back issue. To see Batman larger than life kneeling above Robin wailing in anguish! What is this miracle. You with your DCU and ComiXology with instant access, bah! Graphic novels everywhere! You don’t realize the land of plenty you live in. This was heaven!