The management, the government and the eventually the tax payers.
If the government wants to add a small change to the tax code, if it's not an open source software, they'd have to hire the same company that wrote it in the first place. That's when the companies tend to jack up the prices to crazy numbers.
I have personally witnessed companies winning the initial government contracts by undercutting everyone and then charging them 10X for even the tiniest of modifications. Some times the companies even flat out reject the future contracts because they are stuck with a better project elsewhere and the government is stuck with useless old binary.
If the server side software is open source, depending on the policy, you can also submit your changes to that software that lets you submit your taxes with your own python script.