We have all been there, rage quitting a website because we couldn't complete a purchase. That's poor UX. Not only does poor UX stop people from converting, but it can have a wide ranging impact on your brand name too. Furthermore users who enjoy the process of making a purchase as well as the good they receive from that purchase will want to return to do it again.
UX is, simply put, the experience a user has when they interact with your product, website or app. Every business has different requirements, and their users have unique needs. This means the UX for each product, website or app is also individual to it. The optimum experience for users can be created through exhaustive research, wire-framing, prototyping and user testing.
Users who enjoy their experience are more likely to continue to repeat the experience. Users who don't, won't. To increase retention, UX is crucial.