What event in the development of the brain leads to the need for synaptic pruning?

It for the  Regulatory and regenerative purpose. The motivation behind synaptic pruning is accepted to be to expel un-vital neuronal structures from the mind; as the human cerebrum builds up, the need to see more unpredictable structures turns out to be significantly more relevant and less difficult affiliations framed at adolescence are believed to be supplanted by complex structures.