Not all users will complete their purchase on their first visit when operating an eCommerce store. After filling out their details, some will step back before pressing the “complete purchase” button. At this point, you have their email, so you can easily send them a notice to encourage them to complete the purchase. You do not know the initial reason they left, but your email may be a reminder/follow up that they forgot to buy stuff from you.
You can raise your potential sky-high with strategic email marketing, so do not neglect it.