In the online business, cart abandonment is one of the major problems that the site owners are facing. Consider this scenario: A customer visits your Magento store. He goes through the products you are selling and like them as well. He likes the products so much that he starts adding them in the cart. The customers move forward to make a checkout, after filling the normal details.
But then the worst thing happens, they get to the final stretch and they get cold feet. They close the tab and leave the store. The products are now lonely waiting for the visitor to complete the checkout. The cart has been abandoned.
In the brick-and-mortar store, it is rare to see, as customers face embarrassment in front of the store owner to visit and not buy something. But there is no such pressure in online shopping.