It's a sort of fluid evolution where I can always be in control.
It would be a hard time doing that via visual programming.
Also you could have a timeline that shows you what got changed instead of comparing two text files. There can be lots of ways to achieve the same goal, understanding what changed and how, than just diffing two text files.
Don't you really want to do more meaningful comparisons though? I think text is a poor form for versioning and comparison because you always have to mentally extrapolate the textual diff (lines added/deleted) to the syntactic diff (functions added/deleted/modified) to the diff in effect (which logic in which systems is modified). There is no reason other forms of programs cannot also do diffs, that could be more useful.
Visual does't have to mean flowchart boxes. Even a tree structure might be incrementally better than plain text.
but there is no inherent reason why text is better at this than visual programs. it's just that people haven't done it, which is the issue. everyone just assumes text is the natural way to interact with a computer, but yet most people forget they went through a lot of training, formal or informal, to get to that perspective. and one could argue that comparing text is not as good as comparing the differences between visual programs because the latter actually contains structure and relationships of how things are called more readily.
what you describe with two side-by-side windows of the same VI (i.e., function or code block or whatever) is exactly something that i have done with labview. a better visual comparator is not beyond reach.