Menu Aggregator is a "Squisher" function. There can be cases where exactly the same line items passed to us from POS may be treated differently and it may lead to an erroneous behaviour.
For example, a guest orders 2 units of a menu item named "Burger" with ID 5001 and type 'M' but eventually cancels one of the two. This cancellation leads to the addition of an item say "Cancelled Burger" of Type D i.e. discount albeit having the same ID 5001. This is because the cashier doesn't want to bill the guest of the same. In this case, erroneous behaviour such as the running of a BOGO offer or something else may happen.
To avoid this, an admin can use an aggregator function on Menu Item ID(in this case) to add all Menu Items with the same ID. In the case mentioned above, the final quantity of the Burger will turn out to be 'One' thus preventing any erroneous behaviour such as wrong redemptions etc.