According to the BABOK® v3 Guide, the purpose of Plan Stakeholder Engagement is as follows:

Here is a football analogy: a good way to think about this task is to consider the stakeholders as members of a football team. Every team member has a different role to play, directly or indirectly, but ultimately, everyone has the same goal. Some play positions that are only required for part of the game and other team members stay for the whole duration of the game. There are stakeholders who don’t actively play in the game but they contribute by bringing water to the players or by putting the score on the board. The business analysis team needs to understand who all the different stakeholders are and understand what type of role they play within the initiative’s context.