See the wire? That connects to the ground for your house's electrical system.

You must make sure your house ground is good in order for there to be the potential (this is a pun...) For good grounding.

There's too much annectodal evidence of better sleep, less inflammation, etc. for it to be completely bunk.

Walking barefoot on the dirt IS proven to have interesting effects on blood, so, this tracks with the grounding mat doing things that should be beneficial to any mammal.