Why do breeders look at pedigrees before breeding dogs? Or in different words, how do pedigrees play a part in breeding dogs?

Pedigree's in breeding dogs show the history of the dog and there traits. With a pedigree you can find the disease a dog may succumb to from it's ancestors or physical features.

With pedigree dog breeders can also find out whether a dog is purebred or not purebred.

Explanation:

the unequal cytoplasmic division that  results  in  one egg  and three polar bodies ensures that a fertilized  egg has  ample nutrients  forits journey  to  the uterus.