Simplified Chinese is the result of official reforms in the People's Republic of China, especially lists issued in the 1950s and 1960s. The goal was fewer strokes for many common characters. Some forms already existed in handwriting. Some mergers combined more than one Traditional character into one Simplified shape.
Singapore later adopted Simplified characters for most official use. Taiwan did not. Hong Kong and Macau kept Traditional characters in public life.
Not every character changed
Plenty of characters are the same in both systems. Conversion tools still have to walk the whole string, because the ones that differ are mixed in.
Why converters exist
Once two official sets are in daily use, publishers need mechanical help. OpenCC is one of those tools. It does not replay 1950s committee debates. It applies dictionaries. Read Simplified vs Traditional for usage today, and why Traditional is still used for the other side of the story.