What is the worst scenario for an online shop?
A unique product at an attractive price meticulously planned advertising campaign, clever message, brilliant creation, big budget, high CTR and... an error in the buying process that makes it impossible to place an order.
Of course everything was checked - and even by a few people - just before the campaign was launched. But nobody is able to test cart functioning all the time 24/7. Well, unless you have a superhero.
A dedicated monitoring system may do shopping in your store even every minute. It will add an indicated (or randomly chosen) product to the cart, place an order and even pay for the shopping if you want that. Along the way, it will check whether it received appropriate messages in its inbox.
Of course a small modification will also have to be made in your shop during implementation. It is about that test orders need to be recognized and not accepted for processing.
Read a short story on how e-commerce monitoring can be implemented.