OP is not a boarder, she’s an employee who also boards a horse and since she’s never seen them used and unless she’s got another position lined up, would be wise not to jump to conclusions.
The plastic office mat/carpet runner type mats with protruding, rounded nubs or rounded, very dull “spikes” are pretty common in serious jumping barns but seldom seen because they don’t use them all the time and sure as heck don’t use them in client lessons, with Green horses or even with every horse. Used occasionally they can sharpen a seasoned horse. Used more often, the horse just learns to not care and it doesn’t work. Used when bad riding or physical problems are causing the rail dropping, they don’t work and can get a horse stopping.
I’m NOT talking about the strips of wood with sharp metal nails called carpet tack strips. I saw one of those, I be outta there in a heartbeat. Even the plastic mat wraps, if I saw them out in the ring all the time with many working over them, I’d be out PDQ. Laying around in the back only used occasionally, ? Doesn’t bother me.
It’s a way to “get them a rub” without harassing or hurting them. As the jumps get higher and the horse more seasoned, that’s a tool for appropriate, occasional use. As are heavier, squared rails. Of course Eventers have no need for keeping their horses neat and tidy over fences, their CC fences are solid and keep the horses interested. Losing focus there is more then the thud of a falling rail.