Help in QUERGRID problem

I have been figuring out the solution of the QUERGRID problem of the codechef september challenge 2017. I have gone through the editorial many a times but i was unable to figure out a question. A question is haunting my mind for long. I have tried. but couldn’t got to the answer to my question.

I need to know why does the recursion i.e divide and conquer takes place while finding the shortest path tree… Isn’t it enough that the shortest path tree exist in the first shortest path tree. Why do we need to combine the solution… Only here i am lacking my intution.

Please help me regarding this. Thanking You in advance…