Do I need a mattress cover?

Do I need this mattress pad cover? I've heard mixed opinions. Some people r saying just put the sheet on the mattress which is already water proof. What is the benefit of this extra pad? Are there any risks? It says chemical free and organic. If it's not necessary and more of a hassle to clean instead of just wiping down a mattress I'd rather return and put the money towards something else we still need. Help!