Mattress topper on top of Zinus mattress

[Igor Stankevich]

I see quite often investors recommend a mattress topper for and STR. So i got 12' Zinus so far, and thinking whether we need a topper on top of it. Could someone please clarify why a memory foam on top of another memory foam can make a difference?
Also, would i need the 15' encasement in this case?
I personally used topper only on top of a bad mattress, but if Zinus is so good as everyone says - why the topper may still be needed?