When you teach computers, these industry certifications are something that you often speak about with your students. Rather than just write "I'm proficient with Microsoft" passing these exams give you verifiable proof that you are indeed proficient with the software. The company that runs the exams on behalf of Microsoft works with many schools to become verified testing locations and offer site licenses rather than individual test fees. This allows schools to offer the exams and certifications to their students during computer courses throughout the curriculum.
By finally taking the exams for myself, I learned a lot about the certification process and requirements. I also realized that I either know more than I thought about Excel, or the expert test which I didn't take is what I really think about when I say "I'm not good at Excel." Next time I find an organization offering the exams for free, I may try the expert level. For now, I'm happy to be able to officially say I'm a Microsoft Office Specialist Associate.