The objective is to keep Bernie from getting too many delegates on Super Tuesday. Her supporters and Pete's are most likely to get together behind Joe and keep him in the race.
For instance, in CA a candidate needs 15% to get any delegates. Joe might have had a hard time hitting that threshold if he, Pete and Amy split the moderate vote.
this -
She needs to transfer many/most of her votes to Biden on SuperT voting.
A very good payoff for her in the wings for doing this.