If by "why" you mean "what is the purpose? "  the answer is not at all clear.   What is clear is that this is how new centrioles are made. 

Before mitosis, the centrioles separate and a new centriole grows at right angles to the "old" or mother centriole.   There is evidence that the mother centriole plays a role in defining cell geometry (check here). 

Each pair of centrioles is surrounded by a complex of proteins involved in the formation of microtubules - these microtubules are the major structural component of the molecular machine (the mitotic spindle), responsible for providing each daughter cell with a complete set of chromosomes.