The proper expotential either + or - for your brand Tx will provide reduced non proportional response near the stick neutral but will provide the total max throw at full stick.

Sometimes this is called soft center. It is benefitial in making smooth control possible on some types of aircraft.

Futaba and Hitec you will want negative expo. Jr you want positive expo and other brands you will have to read your manual. In any case you will never want the 'wrong' expo for your brand radio as this can and will make the airplane possibly even unflyable.

John