This tool is a wrapper tool used to select between different Qt versions. Of course, the first and easiest example is choosing between Qt4 and Qt5. But it doesn't end there: it can also be used to select a user's build of Qt.
To experienced Debian users, at first sight, it might resemble Debian's alternatives system. But it goes much further than that, allowing users (not sysadmins) to decide their defaults, and even adding new ones, user-wide. All this can be done using different methods like command line arguments, environment variables and configuration files.
Apart from all that, this is the recommended upstream way of managing Qt, being picked up by several (if not all) distributions, so it can easily be supported by upstream in their documentation.