This is a good question!First of all one needs to grasp the concept of ambassadors and envoys: The former are meant to be permanent representatives, while the latter are usually considered temporary, and meant to negotiate on a single issue or where formal representation is politically difficult (ie. when negotiating with a not formally accepted international subject, such as an unrecognized government or organisation).The legal underpinnings of international law obviously change much across history, as before the birth of the modern legal world after the 1648/9 treaties of Munster and Westphalia, international politics was more complicated and far less rules based in the absence of globally accepted legal principles.Technically, ambassadors and envoys were sent off with a mindset of what they were supposed to represent or accomplish, and keep their home country informed on proceedings abroad. Depending on the individual issue ambassadors or envoys might be endowed with rights to negotiate, but international treaties that had to be subsequently signed by the sovereign (government, parliament etc) would therefore always take their time. For practical matters, until subsequent formal treaties could override any ambassadors' guarantees, an ambassadors' or envoys' written promise could count in international law (when and if that existed) as a preliminary agreement. Ambassadors therefore could actually make a nations' policy independently, sometimes without detailed management from their superiors (this was particularly often the case for British consuls abroad).Safe communication between diplomats and their governments were therefore always crucial to conducting successful diplomacy. In today's world, obviously the importance and role of ambassadors has much weakened, as the "other" sovereign is now just a phone call or teleconference away.There are numerous examples from the age before the telegraph where ambassadors negotiated in bad faith, were misinformed or misunderstood their home governments' intentions, or otherwise miscommunicated, and their statements later had to be quietly retracted. But it was the best practical system available for most purposes, and much better than not having diplomats present to defend your nations' interest abroad. Ambassadors played a crucial role in the early modern period, and their influence in international politics much depended on their brilliance and capabilities (Jefferson in Paris is a classic example). Even today though ambassadors' signals can have far reaching consequences (such as the infamous "misunderstanding" possibly created by the US ambassador's remaining indifferentin a conversation with Saddam Hussein when asked how the US would react to an Iraqi invasion of Kuwait in 1990).