Horses have a much lower impact on the land than heavy machinery causing less and shorter term damage to soil structure. 

It evokes a certain nostalgia for a simpler time, it embodies bucolic life. For those of us seeking to improve our sustainability, they are essentially tractors that run on grass not gas. 

In this busy modern world where keeping a low carbon footprint is important perhaps this olden method can be seen as one of the most environmental ones and the only footprint we leave is a hoof in the soil.