Over the (hopefully) long life of an organisation, the nature and size of the membership is likely to change considerably. Newer organisations tend to attract different members from more established ones, and an organisation might attract different types of stakeholders throughout its lifecycle. Understanding your community better can help you to grow or to maintain your numbers as things outside your control change around you.
Any change in membership can provoke you to consider why this might have happened, and what this means for you as an organization moving forward. You might have lost members or gained many members recently – each variation has impacts on your organization.