What was reason for using material to cover the rear spring!?

[Ayers1]

I'm working on a late 31 town sedan and when I bought it, it was missing the spring cover under the rear seat. I thought that was an odd item to be missing but what the heck. When I started looking for a cover I found out that material was available as a cover. Did they start doing this because it was cheaper, easier, or less labor intensive? I assume there's a special frame that goes around it but I made my own.