Typical use cases are:
-) Let users add a custom donation amount the product's price
-) Let a user completely define the price of a product (by setting the product's price to 0.00001)
-) Let the user suggest a price for an item. (the payment method should set the status only to "Confirmed by Shopper"). You can then either cancel the order in the backend or accept it and send the payment information to the shopper.
-) Let a user bid for an item (use a standard payment method that does not require any online payment, and set the submitted order only to "Confirmed by Shopper"). Once all bids are in, you can choose in the VirtueMart backend which of the orders you want to accept.