Countermarked coins

Hi Guys,

Yesterday I received  a Brazil 1828R 40 Réis (countermarked 80 Réis), https://en.numista.com/catalogue/pieces3974.html I bought it because I didn't realise it was stamped 40 (which was off centre), so it looked like it had a weird spiral design on it, which I quite liked, thought it was odd.

I've not found a great deal of information on why they countermark coins, other than revaluing currency and changes in inflation. Looking through the catalogue, it seems that the 80 Réis coin has been countermarked to both 40 and 20 Réis. This must be massively confusing because wouldn't it be possible to have a pocketful of coins, all the same size, shape and weight with 3 different denominations?  Was this common practice? I remember reading that banknotes can be overstamped, but didn't realise they did it with coins.

I've not seen an 80 stamped to a 20 but I've got a 40 stamped to a 20 and have seen 20- 10.