In bug tracking tools (and probably other similar applications), tickets are prioritized based on discrete categories rather than on continuous values.
This can produce strange and counter-intuitive situations. For example, if the same developer has twenty tickets assigned to him, and three of them have immediate priority, what does it mean? Which one, among three, is more immediate than others? Which most be solved first, taken that all require lots of work? Or must the developer be a three-headed dragon to solve those three tickets in parallel?
Instead, continuous priority indicates that the third ticket must be solved before the fourth one but after the second one which is itself less urgent than the first one. Everything becomes easy: you start from the top, and you go down until you solve the last ticket, those tickets being ordered through a drag-drop by the project manager.
So is there a reason explaining why bug tracking tools use discrete categories, or is it just because it was "always done like this, forever"?