This symbiotic relationship provides support for the theory of endosymbiosis, which states tha

As the question is incomplete, let's discuss about the theory of endosymbiosis generally.

The theory of endosymbiosis states the eukaryotic cells developed from prokaryotic cells.

This theory explains that organelles like chloroplasts and mitochondria existed as individual prokaryotic cells before. The larger cells engulfed the smaller prokaryotic cells as a result of which eukaryotic cells were produced. The organelles like mitochondria have their own DNA and can produce their own proteins which support this hypothesis.