The instrumental case is used to indicate the instrument/object with which an action or state of being is performed. For instance, when you go to work "by car", car is instrumental because it's the 'instrument' used to perform the action.
I've seen the following sentence, however, and can't make sense of why "brother" is instrumental:
"I go for a walk with my brother./Idę na spacer z moim bratem."
The verb iść already implies that the subject goes on foot (and bolstered by "for a walk" anyway), however, I don't understand either grammatically or morally, how is "brother" an instrument with which the subject goes on a walk. It seems to make no sense, "brother" does not help the person go on a walk, they're just an accompanying element, the means with which the subject goes on a walk is already implied in the verb itself. So, to me, I fail to see why is the instrumental case used in this sentence.
Can anyone clarify?