There is a long standing desire to allow developers to extend edx-platform by providing plugins, rather than by modifying the platform's code directly. XBlocks provide this ability for new types of courseware components, but courseware is only one of many ways in which developers need to extend the platform (see Feature Plugins for edX Platform for a detailed enumeration). UI Blocks are a proposed mechanism to allow developers to create pluggable user interfaces and install them into edx-platform or other such code bases.