Are the holes in the plate filled with concrete - or is there a void in the concrete underneath the holes?
Just spit-balling here, but I'm thinking that it could be intended to be used to scrape the mud off the bottom of your shoes before you go into the house - maybe?
My mom lives in southern Arizona where they have some REALLY sticky mud that cakes on the bottom of your shoes, and just about everybody has some kind of home-made "shoe scraper" device located fairly close to the entrance of their abode.
Other than that, I got nuthin'