Earlier quoted context omitted.
The point of the secret ballot is to make it very hard to sell votes, because you can’t prove how you voted. (Vote by mail is not a secret ballot.)
How is vote by mail not a secret ballot? I live in CA where we have had mail-in voting for some time. As far as I know, I don't have anyway to prove to someone that I voted a particular way.
“For convenience,” “to make sure everyone gets their vote in.”
The difference is there is no designated, guaranteed-private voting booth for the act of voting.