Xeno farms can grow in atmospheres that are toxic to regular farms, and theoretically can grow better than regular farms in strange temperature ranges (as they're native to whatever temperature ranges exist on the planet).
It depends on how hostile the local environment is to Earth originated life forms. If open air terrestrial crop farming is going to result in local circumstances destroying the harvests but xenoflora will thrive in that environment and the produce is nutritious it's more effective to use xeno farms.