Business Analyst is the person who is responsible to analyze and understand the business and be experts in the knowledge of how it works. Either they themselves become subject matter experts or keep close relationships with subject matter experts to collect information and discuss ideas of improvement. Business Analysts are generally included into projects, as they provide very important guidance into changes requested by or needed by the project.
In projects, you may find the business analysts also helping with requirement gathering, analysis, documentation, verification and even negotiation. Sometimes (and unfortunately) business analysts could essentially end up becoming only documentation specialists - drafting documents such as Business Requirement Document (BRD), Functional Specification Document (FSD), Charts and Drawings, and User Stories. If used correctly, business analysts could tremendously help towards the success of a project and an organization.