> And this is useful when writing code how?

It's useful for general text navigation, and writing code is an instance of that.

> I'd use such a feature if I had to fill a table, and wanted to move to the same column on a different row. But with proportional fonts, the columns don't line up. So yes, it can move to the same x coordinate, but why is this useful?

"Doesn't work well for use case X" != "not useful [in general]"

If your text is laid out assuming fixed-width fonts, and it's important for you to move to the same column in a different row, then just use fixed-width fonts and the method I described would work as you expect.