Posso pensar em duas boas razões
Ele era muito perigoso para ser deixado sozinho
Harry e Ron inicialmente foram para Gilderoy Lockhart para obter sua ajuda. Após a confissão de Lockhart de que ele era, na verdade, completamente inútil, o professor tentou apagar suas memórias.
“Awfully sorry, boys, but I’ll have to put a Memory Charm on you now. Can’t have you blabbing my secrets all over the place. I’d never sell another book —” Harry reached his wand just in time. Lockhart had barely raised his, when Harry bellowed, “Expelliarmus!” Harry Potter and the COS, ch. 16
Harry e Ron então se sentiram forçados a desarmá-lo, essencialmente fazendo dele um inimigo e seu prisioneiro. Eles não podiam simplesmente deixá-lo sozinho no quarto naquele momento; eles foram forçados a trazê-lo para evitar serem atacados novamente.
Protetor de carne
Obrigue-o a "chutar a porta" e tome o peso de qualquer coisa desagradável que eles estejam enfrentando.
They marched Lockhart out of his office and down the nearest stairs, along the dark corridor where the messages shone on the wall, to the door of Moaning Myrtle’s bathroom. They sent Lockhart in first. Harry was pleased to see that he was shaking.
...
“Well, you hardly seem to need me,” said Lockhart, with a shadow of his old smile. “I’ll just —” He put his hand on the door knob, but Ron and Harry both pointed their wands at him. “You can go first,” Ron snarled