Diffusers and lenses play different roles, in order to fulfil different requirements. Here, you can read the fundamental difference between diffusers and lenses - and the range of environments in which they are applicable.
A diffuser typically sits over the light source and the space being lit. As the light hits the diffusers, the light beams are scattered, resulting in a soft, widely distributed light.
The purpose of a diffuser is to create a lit environment with minimal shadows.
Commonly seen in the home and hotel in decorative fixtures, diffusers are also applicable for commercial applications, offices and educational environments, using the soft light to create comfortable working spaces.
A lens is designed to control and focus the light from the light source (such as LED chips) to direct it into a focused beam pattern. Lenses are beneficial in retail environments, making use of shadows and brighter areas, to draw customer focus towards certain products and items.
They are also used where light is required to travel a distance before it lights anything; shopping malls, airport terminals, sports stadia, warehouses and factories all have the same requirement for directed light. A diffused light would waste energy because the light would be spread too broadly around the space.