In a recent episode of Countryfile [1](popular UK BBC1 program on the country and farming) the resident farmer was mentioning both crop rotation and the issue of black grass invading his wheat fields.
The black grass was slowly out competing his cash crop and would difficult to remove (perhaps taking the entire field out of any production for a year or the use of chemicals).
Having a horde of little robots might greatly assist in manually keeping other plants at bay without indiscriminately affecting the field with chemicals (extra one-use expense, robots would last many years).