One of the problems I have on my property out here is burrowing critters. We have voles, rock chucks, and gophers - at a minimum. There might be more I haven’t found yet. I’ve attempted to deter them from areas we care about (like the garden) with some little solar “burrowing critter” emitters. In theory, they make enough annoying noises under the ground that the critters just go somewhere else.
It’s likely round because with injection molding round items are cheaper (tools & molding).
FYI - every scientifically valid test of ultra-sonic pest repellents so far has shown they only provide temporary relief. Eventually the pests get used to the noise and come back, for a while the US FTC was going after the makers who didn’t clearly say they were for temporary relief.
I’ll add, for anyone looking at these, that they died after about a year - every one. None of them make any noise of any use anymore, and I’m not sure where the failure is.