Ellipsis omits words that are clear from context.
Omit the understood
Full: she can play the piano, and he can play the piano too.
Ellipsis: she can play the piano, and he can too.
Natural speech
I wanted to help, but I couldn't. The missing help is still there in the listener's head.
I wanted to help, but I couldn't help. — heavy
I wanted to help, but I couldn't. — yes