Walls have sub-layers that have a big influence on flow. In order to have an adequate numerical resolution of the solution, a very fine mesh is needed.
Adding a very fine mesh will increase the total mesh count. Thous, making the simulation computationally expensive. Therefor, adding boundary layers may be a more efficient way of capturing gradients on walls.
The model below shows Flow on a flat surface.
The model below shows how smaller elements capture boundary layers, which in turn can result in a higher mesh count.
Using layers can capture the boundary layer as shown below.
And here, We see that without any wall refinements, boundary layers become thicker, which may result in inaccurate results.