When you have to much of a commodity in circulation it tends to lose it value because it common. It will be now wise to witdraw it supply such that it becomes less common,when this happens the value increases because it is not as common like it was before and this automatically causes the price of that commodity to rise. Coin Burn reduces the number of coin in circulation in order to achieve the same thing I just explained with my example. Bitcoin has a particular number of coin in circulation that it can exceed just to maintain it value. Same is what we are trying to achieve with steem although the effect will be seen on the long run and not immediately.