I assume you're speaking of KV63. It couldn't really be called a tomb, being a small chamber located at the bottom of a shaft. It was, quite obviously, always intended to be a "catch-all" for various items. You're right, though, in saying that there were 5 coffins in the chamber. They were quite a mystery, really. The inscriptions on all had been blackened out with pitch. Efforts are now being made to try and clean the pitch off enough to be able to read any name that might be on them. In style, they are Amarna period. But to whom they belonged is unknown. There was also a quantity of large jars which contained used natron, in which there were particles both of animal and human remains. As far as I know, there were insufficent remains to recover any DNA.
As to why, it joins the ranks of unanswered questions that abound in ancient Egypt. The used natron was, mare than likely, considered sacred because it was used in mummification, and could not be trashed, but had to be kept. I understand there was also a mat from a coffin found, that had the imprint of a mummy, so it was taken from a completed burial.
Hopefully, the expedition will be able to tell us more of the details.