Setting this will limit the number of products that can be added to a single order.
For example, you might want each order to only contain a maximum of 20 products so that its fair for everyone buying your tickets and a single order can't buy all the tickets in the inventory.
This does not prevent a person from checking out multiple times with the same order details or credit card. Therefore it's important to consider this limit carefully and if in doubt set a high number (eg: 50 or 100).
Setting this number too low, for example, '1'. It will force people to checkout multiple times if they want to purchase on behalf of colleagues - this will cause duplicate order data that will become challenging to administrate. In addition, it's frustrating for the user and may lead to order abandonment