Basically it's Cass's first encounter with the Bat-team and she can't speak properly. Babs is teaching her how to speak. According to the novel, Babs is also the one who christens her "Cassandra" as there's no other way to address Cass at that point in time. (She can't speak, therefore can't say her name).
(Tho I'm not sure if Babs giving Cass her name is regular continuity??)