The concept of IMO was born after the RMS Titanic disaster. By modern standards, Titanics design made her appallingly vulnerable. Her "watertight" bulkheads, by design, did not extend all the way to the overhead because the engineers, in their infinite wisdom, calculated that it was impossible for the ship to take on a trim or list sufficient for water to cascade over their tops if the bulkheads were of a certain height.
When Titanic struck the iceberg, these calculations were proven dismally incorrect. When people began abandoning ship, it became obvious that not nearly enough lifeboats were available. Many lives and much money were lost in this tragedy.
Up until that time, each nation made its own rules about ship design, construction, and safety equipment. The "International Maritime Consulting Organization" was formed in response to the Titanic event, but was "put on the back burner" when World War I broke out. After the war ended, IMCO was revived and produced a group of regulations concerning shipbuilding and safety called "Safety Of Life At Sea"..."SOLAS". Through the years, SOLAS has been modified and upgraded to adapt to changes in technology and lessons learned.
IMCO eventually became IMO. IMO regularly enacts regulations which are enforced by all signatory nations. Class Societies and Recognized Organizations survey ships regularly to ensure compliance with specific laws applicable to each individual ship. Port State Control authority was enacted, allowing such agencies as the US and British Coast Guards to inspect foreign flag ships calling at ports of the many port states. Memoranda of Understanding were signed by some countries unifying Port State Control procedures among the signators.
Of course, the numbers will never be known, but IMO has saved countless lives, dollars, and environmental disasters throughout the years.