With certification, Pilates becomes more professional. This is good for Pilates as a system of exercise, and therefore good for all present and future Pilates students who need it to help them improve or regain their health or body functionality.
First of all, anybody who desires to teach Pilates on a serious level should get at least minimally certified. When you're starting out you won't qualify for a serious, indepth certification, because those require you have many hours of classroom instruction under your belt.
The first level of Pilates certification is as a mat instructor. This is generally available to any and all Pilates students who are proficient in the basic Pilates mat exercises.
Of course, it should be assumed that you have been taking Pilates classes and therefore are familiar with them. If you haven't and you aren't, what do you want the certification for.
Some healthy and flexible adults, especially those with experience as dancers, gymnasts or yoga students, can probably perform Pilates on the mat well when they first attempt them. But of course they still need to actually know the exercises.

If you take instructor training for Pilates mat certification, you'll be taught all the basic exercises and how to modify them for particular students, such as those who are pregnant or injured.
Typically you must also pass a written test and do a live demonstration of your Pilates teaching skills.
The instructor class may take place in one or two intensive weekends, and cost from $100 to $750.
Of course, nobody can guarantee a job after you are certified. That depends on job opening in Pilates studios in your area. But if you're certified and your competition isn't, you obviously have the advantage. Certification should be a minimal requirement for Pilates teachers applying for jobs at Pilates studios.
Some of the "big dogs" such as The Pilates Method Association and the New York Pilates Studio seem to focus on the upper level certification rather than on basic mat work.
The American Fitness and Sports Association offers a "certification" test you can take online, made up of just under 100 true or false questions. You don't have to pay unless you pass. I can't see what value this is.
LindaFit Pilates offers periodic workshops that give intensive and comprehensive training to a small class of students.
Stott Pilates offers a full range of Pilates certification training, starting with mat work on Level 1.
Balanced Body University also offers the full range, beginning with the mat.
You can get a Pilates mat certification from Pilates on Fifth, in New York City.
If you know the owner of any Pilates studios, ask them what kind of certification - Pilates they look for when hiring a new Pilates instructor.
Next: Pilates Center -- find a good place near you.