A brainstorming session is good when you have;
Very few ideas, or
Too many ideas.
When you have a goal to achieve or a problem to solve, but it’s not clearly addressed and you don’t know what kind of steps to take, brainstorming is useful. You may have no or very few ideas, then get some more ideas from a brainstorming session. On the other hand, you may be overwhelmed because you already have too many ideas. In which case you forget them all, reset your mind, and brainstorm from scratch.