Hi EZ4HZ,
The main reason for a topper cover is top protect the latex from oxidation and other harmful substances.
Latex can oxidize faster with exposure to air and ozone and ultraviolet light along with other substances that can damage it (see here) and it’s always a good idea for a latex topper to have a dedicated cover to give it the best possible protection and to help maximize it’s useful lifetime. It can also help protect the topper from tearing when you handle it. A mattress protector or sheets would offer “some” protection but it’s a matter of degree. There is more in post #3 here and post #3 here that will give you some sense of the effect of different types of covers on latex.
Phoenix