Direct Observation allows the job analyst to see (and in some cases experience) the work environment, tools and equipment used, inter relationships with other workers, and complexity of the job.
Direct Observation of incumbents may be necessary to support testimony if the incumbent or applicant for the job has sued the employer. A Job Analysis is necessary to support personnel actions that were taken. However, the job analysis may be of limited value if the job analyst has not seen the incumbent perform the job. In other words, relying solely on the incumbent's description of their job may not withstand scrutiny in a court of law.
Testimony about jobs personally done is direct testimony and not subject to hearsay rules.