Let's say you write an app in Qt, Ubuntu 14.04 (LTS) has Qt version 5.3 but you want to use this cool new feature from Qt 5.7, you can pull the Sources and build Qt 5.7 on your computer and get it working. But obviously when you distribute it, other people who don't have Qt 5.7 installed yet because they are on a LTS distro will not be able to use it.