Support is one of the major benefits of choosing to travel with an organization. An English speaking representative should be within easy communication at any time of day or night while you are away. Would you prefer an organization that has in-country support? To have the support of someone who speaks both your native language and the country's language can be invaluable, if for nothing more than your own peace of mind.
What happens if you are ill and don't speak the local language? Would you be able to deal with this yourself? Equally, what would you do if you had your passport & money stolen and the local lingo still sounded alien to you? In towns more accustomed to tourism there will be a ‘tourist' police station, but unfortunately this cannot be guaranteed, nor can the fact that they will actually speak your language.
Finally, many developing nations are politically unstable and inevitably these are the countries that entice adventurous travellers. What happens if there is a sudden political shift and you have no idea what the best way to deal with it is?
All in-country staff should be trained to be able to deal with all the above scenarios, and more, if they arose. And indeed they do, from time to time.