A spinner can improve the efficiency of the prop & engine combo. The spinner hides the grossly inefficient root-transition zone of the prop from the slipstream. This reduces the load on the engine & increases RPM in the working length of the prop -- generating more thrust.

Spinners can be sized up to ~ 25% of the diameter of the prop without causing a loss of efficiency through hiding the functional portion of the prop blades..

So you see, this is about more than just good looks.