The articles of association govern the company. The shareholders' agreement governs how the shareholders coordinate their votes among themselves, who receives seats on the board of directors, who may buy first in a sale and who must or may sell alongside, often referred to as drag-along and tag-along.
The agreement is effective only between the parties. If someone does not comply with it, the vote cast is nevertheless valid vis-à-vis the company. It is therefore usually enforced through contractual penalties or by depositing the shares. Open-ended commitments are delicate. A clear term or termination provision creates certainty.