What are the benefits of whisking when making muffins?

Whisking muffin batter.

... affects the entire leavening or raising process because no baking powder is used....

This allows a greater number of cells to form and the cells contain a larger amount of air.

Loss of aeration results in a smaller product with a coarse texture.

Aeration is only successful if the air remains in the batter until it is baked. For large mixes, stabilisers and emulsifiers (or fats) are added during the whisking stage.

(Source: Cakes > Whisking Method)

Whisking batter makes it smooth, silky, fluffy and free of lumps

I think your question is been answered here

"Over mixing batter forms gluten, which in turn hardens the cake", @Theindependentaquarius.

"There's a difference between "don't overmix" and "don't mix" - you're supposed to mix enough to incorporate, just don't try homogenize it", @Aaronut.