It's complicated. She doesn't have a use for it, but it's also her desire to learn about Earth (and thus English so she can read the damned things) because it interests her that made her agree.
Also I thought that Vesta was getting the kindle >.> (Just that she needs to be taught English as well lol). Also she can't teach Alexander to throw rocks, he doesn't have a compatibility with Earth magic, only Yin. In the universe of Re Zero, you can only be compatible with one magic (Unless they have a DP that says otherwise)
And a reason why Avery can't teach her? Mate, he was like 12 or something at the time. And he hasn't had the need to use English since coming here, so he would be beyond rusty, have little knowledge to teach anyway and his memories are stuffed anyway.