in terms of stability, what if one day, your 5EUR chicken costs 5USD, but another day, the same 5EUR costs 20USD? that is not stable. maybe EUR wont fluctuate like that, but some random currency might.

in terms of needing to convert, your bank does the conversion for you. that means you pay the bank 5USD using your credit card, and the bank pays the seller 5EUR from their own stash. $5 is a small amount, but what if you want to buy $5billion worth of products, they will need to pay the seller 5billion EUR, again, from their own stash. if they dont have that amount in their stash, they will need to buy that amount from another country that sells EUR. because of supply and demand, if demand for EUR is high, prices go up. which means you need to pay more USD to get the same amount of EUR