Leaving areas of natural growth at the edges of pastures help farmers by protecting it's field from erosion.
This is because when natural vegetation is replaced by agricultural fields the soil is exposed and it stars to deteriorate, it also dries.
As a result microorganisms that once helped keep the soil fertile decrease and then nutrients also decrease.
When farmers leave stretches of native vegetation around the field this help reduce the nutrients runoff improving the availavility of nutrients.
Another advantage of this method is that it helps conservate the biodiversity. This is because the field borders provide food and cover for wildlife.