What is the goal of requirements engineering, and why does it put so much weight on achieving consensus among stakeholders?
The goal is to know the relevant requirements, reach a consensus among stakeholders about them, document them to standards, and manage them systematically — all to minimise the risk of delivering a system that does not meet the stakeholders' real needs.
Requirements engineering is a systematic and disciplined approach to specifying and managing requirements, with four goals: knowing the relevant requirements, achieving a consensus among the stakeholders about them, documenting them according to given standards, and managing them systematically. Underneath all four sits one purpose: understand and describe what the customer wishes and needs, so as to minimise the risk that the customer ends up with something they did not want.
Why consensus is central, not optional: a system has many stakeholders — users, owners, operators, regulators, the development team — and they frequently want different, conflicting things. Collect everyone's wishes without reconciling them and you ship a system that satisfies no one, then discover the conflict during acceptance testing, at the far expensive end of the cost curve. Consensus means the conflicts are surfaced and negotiated before the system is built, so there is a single agreed target to build against and to test against.
That also explains the discipline's odd centre of gravity: requirements engineering takes a human-centric view, and its declared target is a satisfied customer — not a technically elegant document. Consensus is simply how you make "satisfied" objective enough to build against.
Tip: Disagreement discovered during elicitation is cheap and useful; the same disagreement discovered at acceptance is a project crisis. Consensus work is not diplomacy for its own sake — it is early defect detection applied to people.
Go deeper:
Project stakeholder (Wikipedia) — who counts as a stakeholder, and why their interests routinely pull against each other.