The role of the project manager is to make decisions to move the project forward.
Therefore the role of the business analyst is to supply the project manager with the information such that the project manager can make the best decision.
To provide the information, the business analyst needs to understand the change that is being contemplated as well as the effect of the change on all the stakeholders. This is the same understanding the business analyst needs to facilitate the change.
It all comes down to discover, analyze, innovate, and communicate.