A fuel tank cap helps your system normalize the air pressure inside the tank, which is a crucial function. The pressure can rise too much without it, creating leakage and other problems. Likewise, if your fuel tank cap is in proper working condition, it will fail state inspections.
It is highly essential to make sure to have a fuel cap as it discourages fuel theft. Without a cap, anyone could draw off fuel from the tank, which leads to wasted fuel and increased costs.