I know it's the same, but for example in regards to tickets or passport they always say, "you must be in a possession of a valid ticket/passport" not just "you must possess", that's why the question.
In this situation, you must not simply own one, but actually have it with you.
A follow up question: what's the difference between "in possession" and "in a possession"? The correct expression is 'in possession of', without an article.