[hollerith]

>why they would be required on single-user computers in this scenario?

Because the single user does not write all the software running on the system. The proprietary software the user downloads could have its own agenda contrary to the user. The open-source software has security holes so that for example if the OSS is being used to inspect a repo downloaded from the net, the repo might contain files specially crafted to exploit the open-source inspection software. Of if the OSS is a file viewer, a file downloaded from the net might be able to exploit the file viewer.