Your comments

Two common usecases for me:

1. Adding additional whitespace for readability. Similar to what you do with quotes or parens, but with spaces instead.

2. Wanting to do multi-line editing from the keyboard but without selecting lines in the middle. This is most common if I need to make changes to repeated sections in HTML or something like that. If I had this feature, I could just create a selection using the keyboard starting at the first spot I need the cursor and ending at the second, split the selection, and have two cursors. 

My only alternative in that case is to use the mouse, which for me breaks context. Split into lines adds the intermediate lines too which is often not what I want.