MOO orders are collected before the bell and matched in the opening-auction at one clearing price. They are not sent to the continuous order-book; they queue for the cross.
Funds and index trackers use them because the opening print is a published, auditable benchmark. Retail traders use them rarely, because the open is the most volatile minute of the day and the auction price can be far from the last close.
Example: a stock closes at 50.00, overnight news hits, and pre-market indications drift to 53. Your MOO buy for 500 shares fills at the 53.10 opening print, not at 50.00. Exchanges also cut off MOO entry before the open, often several minutes early, after which cancellation may be barred.
Related: limit-on-open, auction-only-order