Candidate Pre-screening->

Welder testing can be a great way to sort out resume skills from real skills. It can also be a great opportunity to apply any additional requirements that a company may have as compared to general industry. 

Employee Advancement->

Specialized Tests have been used to select employees with greater skill levels related to test taking than their peers. 

Internal Company Requirements for Their Product->

A company may manufacture a product that is not governed by any specific code or standard. They may have their own internal program for managing welding operations that could exceed many codes or standards. 

Comply with the requirements of an owner/designer/engineer->

A company may do work for another organization and will have to comply with their requirements. Some may be imposed by laws or may just be internal to the customer. 

Comply with law, industry standard, code etc.-> 

These may apply if a company is making specific products or supplying specific services. Understand that when laws apply, a company could make the welder testing requirements exceed the requirements of a code or standard. 

Advertising->

The good ole "Certified Welders" in an advertisement can go a long way.  In my experience, they may have some welders that have taken a test but the range of qualifications that they have come nowhere near meeting the conditions used in production. 

Remember, there is more to doing quality welding than having a "Certified Welder".