A device of literary bluster. When one is unsure about something, or sure about what one has no right to be sure about, one writes 'surely.' Example: "Vallicella links to Dinah Washington here. But surely Peggy Lee's version is better. A voice like no other, and the little piano break at 1:13 is exquisite."
I confess to using 'surely and 'of course' promiscuously.