What can the device do?
How is it useful?
How is it novel?
How do I build it?
What does it look like?
How do I use it?
What is the scope of the device?
All of those should be explicitly or implicitly answered by the description/drawings.
The claims only answer one question which would be something like
What invention am I trying to protect?
The subject matter of the claims must be novel and inventive by itself only looking into the description to understand what the words mean exactly. The question(s) why that is the case are answered by/in the description.
For the edit, please have a look at the relevant law or a commentary, there are definitions and examples as to what a description needs to provide. And a commentary with years of experience from judges/etc. is certainly more than an answer here could cover. (I don't have any English recommendations)