The DID and DSD are both necessary in their own right as they can be meant for different audiences and purposes. DID’s basically outline the high level overview of what the product is going to be like: red, can make sounds, light enough to hold, etc. For those levels of people involved with products who only have time/bandwidth to care about the high level details such as management, this is the document that they see, or at least some form of it (whether or not it is processed into even higher level form is another story). However, a document like this is not specific enough for it to manifest as a product. It does not answer the facets of how: how much, how little, how accurate, etc. That is where the DSD comes in, providing the most technical detail of the product. If it is considered in the design, it must be spelled out in the DSD. This is more useful to those manufacturing and designing the project, as their scope is much deeper within this product. If you were to hand each document to the other party, (DID to engineer, DSD to management) then there would be an uproar: management does not need such a technical document (and may not have the deep technical expertise/product knowledge to know how to decipher the document) in order to make calls on it. Engineers cannot design or manufacture a product without details; with such nebulous understanding of the details, a consistent product outcome is not able to be achieved. So each document is necessary in order to speak to the different people involved with the project.