Both the recruiting company and the applicant benefit as the instrument helps both sides of the equation to see in real terms if there is a “fit”, and to what level.
With the importance of what amounts to the most expensive asset a company has, it is a shame to go through all of the expense and investment of new talent without employing readily available information that is there just by asking.
Take someone who’s learning index and behavioural ranking is not within the pattern for the job, they will always struggle to catch up and remain in step, while someone who ranked higher will be bored and make mistakes simply because they are not working to their capacity.
If in the case where too few applicants make it a shallow pool to draw from, at least knowing more about the person will help to make the most of the person, for their benefit and that of the company.