Mission is the goals for anything and the reason for being exist.
Without mission, the being of things is questionable and out of focus i.e. the mission of a a glass can vary from carrying water but also can transform into self defends tools that alter the early reason of its mission, depending on the users. Mission, similar to "the glass" case, define the purpose of someone, organisation, or something to pursue whether the goals finally reached or not.
For some, mission is already designed and taken for granted bur for some still need to define what is their mission. At any point, the question to be asked is does the mission actually representing what could be accomplished? For those already had mission, the question to be asked whether the mission available relevant to current condition to achieve the goals. For those who doesn't have mission, start asking what is the goals to be achieved and how it relevant with current condition. Mission always referring to what goals to be achieved - the reason of being.
Mission needed  to be stated simple enough to understand, inspire enough to deliver the change wanted, and describes the goals to be achieved.
In term of Capacity Building, having mission is important to know where to go and where to start to work on. Capacity Building works better when mission clearly stated.
Until next time - Be wise, Be successful!