It could be useful for changes in CSV files or text files with table-like formatting where you want to commit the change in just one specific column of a row, but not the other. I could also see it for binary files which aren't structured into lines.

I use it quite a lot to stage a subset of a change, and then drop the rest of it. In that sense, it's like an overpowered undo tool that helps me keep a change I want without having to rewrite the whole thing.