Cost center & profit center ...

hi experts ,

can anybody explain in a brief & clearly why we require to maintain cost center and profit center ?? ....

I only a little bit that cost center controls over all the expenses incurred and profit center is responsible profit & loss a/c . when we procure stock material the cost will be not required ?? only requires for non-stock items & external service ?? ...

and when profit center is to be needed ? & is it mandatory ? ...

thanks in advance ,

regards

arghya