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The main advantage of using a crib mattress protector is that since it is perfectly fitted on the mattress, it does not fold or roll back easily. Thus, even if your baby is lying down, sitting, or standing the mattress, the protector protects the crib bedding.

https://youtu.be/QvTpCQSoQD8
https://www.parentingscoop.com/why-do-i-need-a-crib-mattress-pad/