In which function of management and in what circumstances is brainstorming session mostly used?

Brainstorm should be applied after any feedback for critical/bad cases also to find new innovated solutions/processes for the company/people/customers

Management is POLC -Planning, Organizing, Leading and Controlling.

When you start to do something new, you have to analyze every aspect of that area, like SWOT analysis. It is used in almost every function of Management.

For Planning you have to make analysis based on the input data you have, resources and abilities.

Further ahead if you facing any difficulty in Leading function, start identifying the problem because that is half of your solution. start brainstorming what causes, effects, measures to take over, and control for future.