The peat plates are too big and too thick to use under gravel.
I have frequently read posts suggesting that one put a tiny bit of peat under gravel in planted tanks, for the high CEC value of the peat. The amount suggested in perhaps one handful or a tablespoon or therabouts. Also, that should be moistened, which may require boiling.
If you go to the SkepticalAquarist.com site, somewhere there is a link to a page all about peat. It is used to soften the water, as it binds up calcium (or magnesium, I forget which) and it adds humic acids and lowers pH. It can help to add CO2 to tanks where CO2 is not injected, but this is indeed from a decaying process so you don't want much of that going on.
I have tried each of these things, once, and didn't see a great help in my tanks, but then, I think my moderately hard and quite alkaline water is just hard to adjust or soften without using stronger methods. In the end, I just accepted that I'm not going to be breeding certain fish unless I invest in a RO unit, which is not likely to happen. My tetras are fine in this water in the planted tanks with CO2 injection, the cichlids are fine in straight tapwater.
I have read in books, perhaps old books, of people suggesting that plants be stuck in chunks of peat to be planted in plain gravel tanks. These peat plates would be useful for that, of a square chunk was broken off. I think we have progressed past that now.
This peat is also considered "aquarium safe", for some peat from the garden center might not be. It is rather expensive, though, when you can get a cubic foot or two of peat at Walmart for $3 or $4.