The purpose of cellular respiration is to enable cells to create and use

ATP which is the acronym for adenosine triphosphate. This compound is a nucleotide that can store large amounts of energy in it's last bond. Cells "run" by using ATP as a fuel source. 

If your asking the reason why they did this is if you look at the process at which they did it. The articles of confederation failed as they gave government few powers. So in a basic social contract theory, they gave away a few rights and or freedoms for a mutual benefit. Also written in the constitution there is checks and balances, which was brought up in the federalist papers. This gained support for a new and updated government.