Because I don't have a very good wheel, and it's a bit old. When I center it, if I don't have a 1 or 2% deadzone it's hard to keep it straight. Until I can afford a new wheel, it's a compromise I have to take, giving some stability to straight line driving. It's such a tiny dead zone it doesn't make any difference to the cornering, etc, just stops the wheel sort of wobbling when I hold it straight.Not that I can see.
Why on earth would you want to set up a deadzone in your wheel? That's the one thing sim racers go out of their way to avoid