Why does Amazon use "bar raisers" in the hiring process? What are the potential advantages of using bar raisers for Amazon?

Bar risers are mostly people hired during the recruitment process to interview and vet people applying for a position in Amazon and who do not work there. These people have a vast experience in the labour sector such that they know exactly what to look for in an employee.

Amazon hires bar raisers to analyse each candidate in a recruitment process that takes about 23 hours with each candidate, and they then later compare the notes and pick the employee that best fits.

Potential advantages are

1. It helps in speeding up the recruitment process. Since bar raisers are not Amazon employee their only job is to recruit employees, they have a lot of time to focus on that task and therefore make it efficient and very fast for the company to gain new and highly qualifies for the position.
2. The bar raisers are fair when it comes to recruiting since they don't know ways to have any influence from the already working employees because they are independent workers. Also, employees are picked depending on the skills displayed and after serious discussions among the bar raisers.