Suppose you want to buy something that costs a quarter, and what you have is 3 dimes. If there isnt a 5 cent coin, this creates a situation where you have enough money, but making exact change isnt possible, which while not impossible to deal with is bothersome. If we moved to only dimes and no quarters or nickels, it would never make sense to make a price end in 5 cents, so any price would be a multiple of 10 cents and change can always be made. Alternatively, if you get rid of dimes and nickels but keep quarters, then it doesnt make sense to charge a price ending in something other than .00, .25, .50, or .75, and so you can always make change for those prices with the coins one would have.