Diplomacy is very important to the contemporary international system, especially now that the field is so much more complex than it perhaps used to be.
For example, these days nations that have their own agenda are forced to cooperate to work together on joint business ventures, to ensure security, or to assist in humanitarian situations or to defuse political and regional instability.
To assess the relevance of diplomacy, you only need to look at how many news stories these days feature different nations cooperating, because behind that cooperation is usually weeks of painstaking diplomacy and statesmanship.
And also the rifts and conflicts that occur are also sometimes the result of diplomacy between allies supporting each other in military terms, or the fallout from the breakdown of diplomatic relations over a disagreement.
The current crisis in Ukraine, and the intervention of Russia despite global pressure not to intervene is a classic example of all sorts of diplomacy going on in front of the cameras and behind the scenes.