Why would diplomatic ties be important with adjacent nations? How are they useful?

The function of diplomacy, negotiating the maintenance of relations with another State or body, making agreements and defending a nation's interest in bilateral, regional and multilateral spheres; inform their States of the existing conditions and the evolution of events in other countries around the globe;  represent your country at international events; as well as promoting foreign trade and the image of the represented state.

Diplomatic agents and heads of state need to make decisions at all times which, presumably, will commit them to doing something, but not all of them will be strategic decisions.

The practice of diplomacy has become one of the main tools for States to expand their national interests. Diplomacy is not only the practice of international relations between States through legally authorized representatives, but also the art of negotiation, aimed at defending the rights and interests of a people before foreign governments, and resolving conflicts over peaceful means, through the force of law.

answer; the negotiator was not in full control of his or her actions and hence should not be held responsible;

answer; broad features of society;