UX Designer
UX designer develops for your business the procedure of improving overall user experience with e.g. website, application or app interaction with an aim for higher customer satisfaction. Mainly focuses on wireframing an application and at the same time creates user flow through the structure of components.
UI Designer
UI Designer, in contrast, is an actual process of improving the presentation and interactivity of mobile applications and the web. Mainly focuses on the look of an app or website for better user interaction. Every single visual piece or element that you see on a website or mobile apps such as buttons, page, screen etc., is the UI of that application.
The main goal for the business is to improve sales and growth of the business. Therefore, the UX / UI Designers are the key components for achieving these goals. By hiring UX and UI Designers you will have a better user experience and customer satisfaction which would lead to a higher number of application users and thus increases the chances of higher sales.
https://medium.com/@thinkwik/do-you-know-the-importance-of-ui-ux-development-773eae38436e
https://www.targetinternet.com/guide-to-building-interactive-emails-top-5-resources/