Hi jarnholt,
I think you may mean a mattress protector (your mattress already has a mattress cover).
Assuming this is what you mean … then yes, a protector is important to protect any mattress (regardless of the type) from the body fluids and skin cells that each of us release during the night, from stains or accidents (which would invalidate the warranty), and to keep your sleeping surface in a clean and hygienic condition.
Post #89 here and the posts it links to has more about the pros and cons of different types of protectors that can help you decide which type of protector would be best for your preferences, circumstances, and budget.
Phoenix