For psychometric assessments to have utility and be effective when assessing people for various purposes, the assessment has to be reliable and valid for the situation.
All personality tests are not 100% accurate and measurement errors from a variety of sources can affect the results. The length (i.e. the number of items) of the assessment affects the reliabilty of the assessment and research has demonstrated that measurement errors are smaller in longer assessments than in shorter assessments. In addition, a larger number of items better represents the abstract characteristics that are being assessed. For example, when assessing personality, one cannot expect to obtain an accurate picture of an individual through a few questions, therefore more items are needed. It has to be noted that after a limit, increasing the number of items will not provide further increases to reliability as other factors such as fatigue will set in.
It is for this reason that good personality assessments will have a large number of items and therefore require some time for the candidates to complete the assessment (usually between 200-250 questions, taking around 30-40 minutes). Psychometric assessments that are shorter will tend to be less reliable and valid. With a large number of items, the reliability of the test will be better and in turn the validity of the assessment will be better too. Validity is all about predicting performance. So with high validity human resource professionals get a higher return on their investment.