When the first plants moved from the water to the land, they needed to be able to reach higher than plants around them so as to get the most light.
This meant that the plants had two mean needs: they needed conductive tissue to carry water to the higher parts, and they needed stiffening to have not bend.
Many animals and plants have stiffening material or organs that give them a framework on which to move. In animals, this gives the muscles something to work on.
Animals may have an exoskeleton or endoskeleton to provide a degree of protection, but also to make fast movement easier, as it gives something to pull against.
Sharks and rays have cartilage and no bone in their skeletons. This provides a flexible skeleton, but offers less protection than one made of bone.
Wood and bone are both two-phase materials, combining a material which is strong under compression with fibres which is strong under tension.
The plants came to use the conductive tissue as high-tensile fibres to provide much of the tensile strength: secondary thickening dealt with the compression.
Bone is a two-phase material, made of mineral material (calcium phosphate) for compressional strength and collagen fibres to provide tensile strength.
Bone is a dynamic material, with small parts of its structure continually removed and replaced, in part as a response to charges generated within the bone.
The pelvis is a critical part of the human skeleton, because it lets us walk upright, and its size limits the maximum possible size of a newborn baby's head.