This is part of a larger government initiative to curb outsourcing of IT projects. Traditionally these sorts of things would be built by large software companies at enormous expense to the taxpayer. Gov.uk has been built by a (relatively) small in-house team, by people who genuinely care about what they are building. They embrace the fact that they are building tools for the good of society rather than just satisfying a contract.
Also as a citizen, I love that I can open a pull request on my government's website ( https://github.com/alphagov/calendars/pull/1 ). We've got the ball rolling in opening up government data on the internet, but this is a great example of how technology can enable citizens to get involved in government.