Board Thread:Questions and Answers/@comment-27176987-20160913054504/@comment-26516215-20170205091409

(Spoiler)

the seraph of the end was made by the Hyakuya Sect and they don't really say why they were made, just that they grow inside a person with the trait like Yu. Based on what Ferid says, they it is for reviving the dead which brought angels of destruction. Basically they are to punish humans for breaking the taboo of reviving the dead and may be related to resurrection. Hope this helps.