An institution receives far more applications than it has positions. A competitive examination gives candidates a common task or set of questions, allowing selectors to compare evidence produced under a defined process. Someone outside the institution's usual network can, in principle, demonstrate relevant ability through that route.
An examination may be only one stage. The UN Young Professionals Programme combines assessment with a broader recruitment and development process; passing an exam and obtaining a particular appointment are not the same event.
What the examination rewards
Imagine selecting archivists through an exercise involving unfamiliar records. If the task samples the judgment the job needs, results can be useful. If it mainly rewards memorizing obscure facts, candidates may learn to excel at the gate without becoming better at the work beyond it.
Preparation opportunities also matter. Coaching, spare time, language familiarity, and access to materials can affect performance before the examination begins. Common questions do not make candidates' starting circumstances equal.
The examination's attraction is that it replaces some personal discretion with an explicit comparison. Its limits follow from the same compression: a complex person becomes performance on selected tasks at a particular time. Where the exam forms part of merit-based selection, selectors still have to interpret that evidence rather than treating a rank as a complete description of capability.