Exponential values changes the stick response curve from a straight line (linear response) to an S-curve.
If exponential value is positive, the curve is flatter in the middle (less sensitive response) and steeper at the extremes. This means you get more precise control in the range around neutral where most flying takes place, while still retaining full throw when needed. Useful when high rates are used.
If exponential value is negative, in opposite the response is faster around middle stick... not recommended for beginners!