If you cross a bank draft only an account holder can en cash the same. It is generally safer than the normal bank draft.
Assume you take a draft for 100 dollars in favour of Mr. ABCD. If you don't cross it, Mr. ABCD can en cash it without having a bank account. However if you cross it the money will go only to a bank account. So, at any point of time you can prove that you had indeed paid to Mr. ABCD. Another added benefit is in case the draft is lost, any body with the same name can claim the cash.
Thus you get two benefits : Added security as well as a proof.