What Is SAPUI5?
Basically, UI5 stands for User Interface using HTML 5. So this is one of the framework that SAP is using to enhance its frontend. It is just like any other HTML5 client-side rendering library. It supports  both desktop and mobile platforms with the help of some set of libraries. SAPUI5 is a library that combines all of those things into a toolkit for building actual Web applications.Applications can be built using SAPUI5 that connect to any type of back-end system.
What is OpenUI5?
OpenUi5, Open source variant, for the developer community of SAP is contributing key portions of SAPUI5 framework as open source code on Github site.
OpenUI5 is the free version available under the Apache 2.0 Open Source license.
SAPUI5 is the version that may only be used by SAP customers with a certain kind of license.
Why to use SAPUI5?
Before SAPUI5, SAP used WebDynpro for building SAP Web Applications, which was not rich in graphics, and wasn’t user friendly. Hence, SAP introduced SAPUI5 late in 2013. Now SAP is promoting the use of SAPUI5 and fiori to enhance its user interface.
I am providing this link, where you will find out the real need of using SAPUI5