When you need high quality (IPC/MIL/etc. standard certified) repair. Small nozzle, fast heating results in localized reflow, so other parts are not reflown.
Aah, that seems fair enough.
I do OK as far as keeping heat in one place with my quick 861(with extra small nozzles, bought separately) it's not common that I reflow multiple parts when I'm focussing on just one. (though tightly placed 0402s can..) But it's a pretty bulky handpiece, and the general handling of it gets a bit tiring.
If I was reworking lots of little parts I'd definitely like something like this one.