Product development is most often a process that in its early stages is dominated by technical considerations after the initial idea is articulated. It is about the dimensions, performance characteristics, functionality, features, technology platform, and so on. Typically the product brief will cover all these aspects, and more, but it will not be enough for the designer responsible for the way in which the products interact with the consumer, and therefore does little for the development of your brand.
At some point, you need to reflect the behavior of consumers as they use the product, the level of engagement it attracts, the way they interact with the features, the situation in which it will be used, what else is likely to be going on around them concurrent with use, the way it needs to look and feel, and many, often qualitative aspects.
It is a common mistake to put a lot of effort into the product brief, but less into the design brief, thus ignoring the most important factor in the whole exercise, the consumer.