Why where coins deviloped as a medium of exchange because other items like cows, grain, and land where more difficult to move from place to place

Money has a guaranteed quality that matches its face value. For instance, grain can be infested with insects or disease, so the quality of a transaction using grain as the payment method will vary. The seller may find that some of the grain received is poor quality even if it’s the right amount. Similarly, a dairy cow that can produce a good quantity of milk may look exactly like one that produces a poor quantity of milk. So the animals don’t have the same value.
The denomination of money down to a coin level means that all prices can be matched. Then there’s the problem of change. If something costs 3 pigs and a chicken, and I only have a cow with me, I would expect “change” as well as the item I’m buying. So the “payment” can’t go ahead, unless the seller has a suitable choice of acceptable change in produce or livestock. Money is lighter to carry around, and you can easily transport money of different denominations because it doesn’t take up much space; and the seller can carry a good supply of money to be able to provide change to the buyer.