Why is the Iranian Rial the least valuable currency in the world?

Iran also called Persia one of the oldest world's civilizations, after the Iranian revolution it becomes the Islamic Republic of Iran. It is the world's 17th most Populus country spanning 1,648,195 km-sq and the second-largest country in the Middle East. 

As we know Iranian Rial is the weakest currency in the world. For simplification Iranian uses the term "Toman" 1 Toman = 10 Rial. When people of Iran say 10 tomans it is meant by 100 Rials. Simply eliminate the last zero from Rial it becomes Toman. In less formal situations, everyone says 5 means 5 thousand Toman, or 50 means 50 thousand Toman. The reason is that Rial it self worth so little. Iranian Rial can not be exported, you can use Iranian Rial only in Iran. This makes the Iranian Rial one of the least Internationally useful currencies in the world.

There are several reasons why the Iranian Rial is the weakest currency in the world. The war between Iran and Iraq. Some threats faced by Israel danger of possible nuclear war. Due to this situation, Iran was restricted to access to the world's market, which caused Iran to lose its economy. Being an oil exporter, Iran could no longer export its oil, which created a critical deficit in its budget. All of these reasons led to the Iranian Rial as the lowest currency in the world.