If, for instance, your 9-speed DA shifters pack it in and you want to replace them with something nicer than Sora, without replacing your whole drivetrain.

Or, no manufacturer sells a really wide range groupset with brifters. It's all hacks, and the hacks have been getting more difficult since the end of SIS. The 4700 shifters are the best STI's still available as a triple and so if you can get it to run an SIS system - be it 8 or 9 - it will allow you to make a really wide-range drivetrain with an SGS RD and run it with brifters.

And, just because figuring this stuff out and tinkering with it is fun.