I think the idea here was that if you are working alone, you have a mental model of what the code is supposed to do. Tests are a way to share that mental model with collaborators.

Also, if you are working alone without automated testing, then you probably rely on manual testing. But that manual testing isn't easily reproducible by somebody else without detailed instructions.

Also the previously stated point about safe refactoring.

I think our general sentiment with this point was "there are many reasons that automated testing is valuable, but that value increases significantly as soon as you want to collaborate on code with someone else."