purpose of making an MOC order

With this type of order (market on close), you participate in a special "closing auction" rather than paying a bid-ask spread as with a normal market order. This can reduce trading costs (slippage).

Suppose you have backtested a strategy with trades at reported daily closing prices. As long as your orders are small compared to market volume, you can expect to get the same execution price (the reported close) whether you buy or sell, whereas normally you'll pay a slightly higher price to buy (the ask) than you receive to sell (the bid).