Don't confuse this certification with being a welding engineer, a licensed discipline of engineering. Welding Engineers are specialized and while there's great demand in some sectors, it's a relatively small specialty.
A Certified Welder, obviously, is one who would do welding. A Certified Welding Inspector has to meet some specific requirements and testing in order to inspect welds under AWS, API, and ASME Codes.
Meaningful and credible certifications are good to have. For example, to find a structural engineer who is also a CWI is very unusual, but you can bet he won't be buffaloed by the fab shop or the erector when it comes to welding. Unlike many structural engineers, he will know the differences and limitations of welding processes, be able to tell a good weld from a bad weld, and gain a little credibility in the field.