Answer;
-Because they provide energy.
-According to the endosymbiotic hypothesis, why did large cells not destroy the small aerobic cells they engulfed because they provide energy.
Explanation;
-The endosymbiotic hypothesis explains the origin of mitochondria and chloroplasts, two organelles contained within various eukaryotic cells. According to the hypothesis mitochondria and chloroplasts evolved from prokaryotes living within prokaryotes.
-The theory suggests that eukaryotes were formed from the union of small aerobic prokaryotes with larger anaerobic prokaryotes. The smaller prokaryotes were not destroyed by the larger cells but instead they became internal parasites that remained surrounded by a vesicular membrane of the host.