They aren't really necessary. The reason that they are included can only be known by Satoshi, and AFAIK, he did not state why he chose to include nBits in the block header (or many other things that are just arbitrary). This is one of the many things that Satoshi chose to do and no one really knows why. It remains in the block header today because removing it would require a hard fork and there really is not much benefit to be gained by removing it.
The nBits field can be useful, and it likely was included as a convenience sort of thing. Instead of having to have the full chain history in order to know the current difficulty, you can instead look at the nBits. But that is just speculation, and full nodes don't use the nBits to determine the current difficulty (except those of the genesis block).