Many civilian jobs have certain professional and technical standards. Obtaining credentials – certifications and licenses – shows that you meet these standards. For example, a welder can show his welding certifications to an employer to document that he has specific skills and knowledge. In the civilian world, credentials may be required for a job, or can make you much more likely to be hired for a job.
For a Marine, civilian credentials are important for two reasons:
The Marine Corps has made them part of its workforce professionalization, so getting certified can help you in your Marine Corps career while you're still in service.
When you transition back to civilian employment, credentials help you translate your military training and experience into something civilian employers can easily recognize. That can help you get hired, get a better job, or be promoted sooner!