Sc.2, lines 244-258 Why does Hamlet urge his friends to keep the news of the ghost secret?

"And bid me hold my peace. I pray you all, If you have hitherto concealed this sight, Let it be tenable in your silence still; And whatsomever else shall hap tonight, Give it an understanding but no tongue. I will requite your loves. So fare you well. Upon the platform, 'twixt eleven and twelve, I'll visit you." ~ Hamlet