For membership organisations
If your company or association is a charity or a not-for-profit organisation, then you are likely to have members rather than shareholders. If your company runs a business, it will have shareholders.
When anyone wants to become a member or shareholder of your association or company, you want them to agree to any existing constitution, articles of association, by-laws, shareholder agreements or other governing rules of that you already have in place. That agreement is the contract between your business and the person joining.
These documents help you record the agreement between your members or shareholders and your company or association.