In grafting, the rootstock of a varietal that is better suited for the soil (or stronger in the environment) is combined with a varietal that grows using a different root system.
For example, we are working with a farmer in Antigua who had a shade grown plot of coffee plants that started dying. He found out that an insect was eating the flat root system of his arabica trees. He decided to take a robusta root system (straight, deep roots) and to graft them with his arabica varietal. It worked: The forest is now full of healthy coffee plants and the insects have to go somewhere else to feast.
Grafting young coffee plants during their "soldier" phase in a research centre in Kenya