This region has always been one of China's major centers for foreign trade, from the days of the Maritime Silk Road through the era of tea clippers and into recent times.
Guangdong province produces about a third of China's total exports; most of these are from the Delta. The region is often referred to as "the world's workshop." The largest product groups are clothing and electronics but the industrial base is extremely diverse and the range of products manufactured is enormous.
In 1978, a small processing workshop making handbags for export to Hong Kong opened in Dongguan. This was the first export processing business in China. From then on, as Deng Xiaoping's "reform and opening up" policies went into effect the Delta underwent absolutely frenzied development, partly fueled by heavy Hong Kong, Taiwanese and foreign investment. There was also significant investment and massive migration from other parts of China.
During these decades, GDP growth in industrialized countries was generally 0 to 4% a year. China managed 8 or 9% overall national growth every year for decades. The Delta was the fastest-growing region at around 15% a year! That is, GDP doubled every five years from sometime in the 80s until 2008. Shenzhen, the showcase city of the Pearl River Delta, likes to brag it is "the fastest growing city in the fastest growing region of the fastest growing province of the fastest growing country in the world."
Part of the "opening up" was creating Special Economic Zones (SEZs) to encourage investment. Two of these — Shenzhen and Zhuhai — are in the Delta. Both have undergone phenomenal growth, from fishing villages in the 70s to bustling modern cities today. Shenzhen is now well over 10 million people, Zhuhai about two million.
The Delta was hit hard by the world economic slowdown and the collapse in exports around 2008, but has mostly recovered.