Validate your network security skills and experience. Demonstrate value to current and future employers. Leverage Fortinet's full range of network security products, consolidate solutions, and reduce risks. As a partner, accelerate sales and offer new services.
What Ramzi said, but also to increase your width and depth of product knowledge. When you work on a product you learn the sections you need to know to complete the configuration that is asked for. With certification you are forced to go into areas you have not used already and also find different and often better ways to achieve tasks you have done. This goal helps to make certification a much more beneficial process than simply to pass.
The cost seems pretty standard for IT exams. If the cost cost is too much, then you are not seeing sufficient return. That is why I posted what I see as value for the exam. Also likely depends on who pays for it and the value they get. Certs also tie into pattern levels and profits so $400 can become a great value if it leads to profit increases.
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