You end up with for example Polish which has has nouns in the instrumental case after the verb "to be", or at least I think it's the instrumental. In such a simple sentence such as "I am a student" why in the world would the word student end up being in the instrumental case? It makes no sense.
From my foggy memory I think this would be "Jestem studentem" as opposed to just "Jestem student".
These are the things that baffle me about cases. Is there any reasoning to how these sort of things develop or is it just idiomatic and illogical like so many other parts of language?