Employers use aptitude tests in order to retrieve additional information from their candidates; information that cannot be obtained from their application forms and interviews. Tests can provide information on an individual's aptitude, interests, reasoning skills, etc.
If someone is applying for a job through a recruitment agency, very likely may take tests with the recruitment agency rather than with the hiring company.
The validity of Intelligence Quotient as a predictor of job performance is quite decent, although varies with the kind of job. While IQ is more strongly related to reasoning and less with personality and emotional intelligence, IQ test scores predict performance ratings in all occupations. For highly qualified activities low scores are more likely to be a barrier to adequate performance, whereas for minimally-skilled activities, athletic strength are more likely to influence performance.
General cognitive ability (EQ + IQ) predict academic success, with the exception that processing speed and spatial ability predict performance on the SAT.
Said all that, it is very interesting that the US military has minimum standards at about the IQ 85 level. There have been two experiments with lowering this to 80 but in both cases these men could not perform their job properly.
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