The website has another message that publically funded software should be open source, but that's not always possible. The government may not have the rights to even do so if they outsourced some of the development. The software is often niche and would not benefit others. In fact it can be a security risk because attackers can look for security vulnerabilities or weaknesses in systems. Attackers have a much bigger insensitive to look for security problems than security researchers because there won't be a bug bounty, it isn't software they personally use, and it could be some rare piece of software no one knows or cares about. If most talented developers are working at private companies, that means that the remaining developers who chose to work for the government are likely more prone to have poor security practices. If attackers know the supply chain of the software they can attack it. If these are open source in the sense they take contributions attackers can contribute vulnerabilities. Open sourcing code is also extra work that has to be done and will make the software more expensive to make and maintain.
>Tax savings
>Similar applications don't have to be programmed from scratch every time
Buying existing software means you don't have to program it from scratch. Sharing projects with other agencies doesn't require open source.
>Collaboration
>Major projects can share expertise and costs.
This doesn't require open source either. The government if they didn't have an income stream from leaching off it's population would be incentivized to figure this out.
>Serving the public
>Applications paid by the public should be available for everyone.
Most of the software will be useless to the public.
>Fostering innovation
>With transparent processes, others don't have to reinvent the wheel.
This is just the collaboration point. The government isn't innovative in the software field.