Back in the old days not backing up your drag when the reel not in use meant constant pressure upon the drag washers and drag pad material, which were not as advanced as the material found in modern day drag pads, in some cases they were made of felt like material, in others made of something akin compressed cardboard, in others of compressed cork, in others it was leather. The strength of you drag was proportional to the thickness of those pads, thinner pads meant weaker and less responsive drag, besides, most of them had wet drag pad system, the pad was soaked in lubricant, the constant pressure caused the lubricant to ooze away making the pad dry and brittle.
In modern day reels the drag pad material is ages away from those early drag pads, most of them are incompressible, but the drag washers and springs no matter how modern and no matter how strong will flatten out with time if maintained under constant pressure. That is why you should loosen the drag when the reel is not in use.