Why friction is necessary for growth

"bone in a healthy animal will adapt to the loads under which it is placed.If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading. [...] The inverse is true as well: if the loading on a bone decreases, the bone will become less dense and weaker due to the lack of the stimulus required for continued remodeling."

[0] https://en.wikipedia.org/wiki/Wolff%27s_law