An organisation must be clear on the purpose for which it is set up. If the organisation is or would like to be a charity then the purpose for which the organisation exists has a special significance.
Organisations are established for all sorts of reasons. Some are to raise money for a particular cause, some to deliver a service or some to represent the views of people from a particular geographic area etc. Organisations should be very clear on the purpose for which they exist and these purposes should be clearly outlined in the governing document of the organisation.
If your organisation is considering registration with the Charity Commission for Northern Ireland, you can find more details on purposes which would be considered charitable here.
Over time organisational purposes may shift due to changes in the organisation's environment. The board needs to regularly review the organisation's activities to ensure that they are within the purposes and that activities haven't drifted away from the reason the organisation was established. The board should see them selves as guardians of the organisation's purposes.