In an episode of Marvel's Agents of S.H.I.E.L.D. TV series, Agent Victoria Hand said to Agent Ward that no agent was that important to the system (At the time, Agent Coulson had been kidnapped and his team was trying hard to find him).
However, during the events of the Avengers movie,
Agent Coulson was impaled and killed by Loki. We later learn in Agents of S.H.I.E.L.D. that Director Fury had him revived via a classified project.
Why was Agent Coulson so important to Director Fury? He was just another Level 8 agent.