Answers for the advantage of the endoskeleton over the jointed exoskeleton will also vary but most will include methods of growth. Support, lever system, protection, blood cell production, mineral storage. Mold for future bone formation. Collagen and minerals. Collagen is strong, flexible, and elastic.
Minerals are rigid.
Without collagen, the bone would be brittle. Without minerals, the bone could be tied in knots. Blood vessels are found in the bone canals. The blood brings nourishment to the cells. Cells, replete with communicating cytoplasmic extensions, are found in lacunae within the bone matrix.