Stoker chose the epistolary format to structure Dracula. Epistolary novels are presented as a series of diary entries and/or letters. By the time Stoker used the format, epistolary novels were an old tradition stretching back to Samuel Richardson's Pamela, or Virtue Rewarded, published in 1740, and yet he updates this commonly used structure by including not only letters and diaries but phonograph recordings, medical notes, news articles, and telegrams. The epistolary structure has many advantages, but the main two Stoker draws upon are its sense of realism and its psychological immediacy.
Dracula's structure allows Stoker to make the supernatural elements of the story seem real, almost documentary-like in effect. In a traditional novel, one might more comfortably suspend disbelief in vampires or other supernatural creatures; by presenting a vampire through letters and news clippings, Stoker makes the reader wonder how one could potentially operate in the real world or how such a creature could even be detected.
The epistolary structure also allows the reader an intimate look into the thoughts and feelings of multiple characters, adding to Dracula's epic scope. The reader gets glimpses of Dracula through the different characters, from Jonathan's time in the castle, through Mina's horrifying seduction, and through peeks at Renfield's madness. All across these seemingly disconnected documents, the reader is able to piece together the extent of Dracula's power. This increases the suspense of the story and the reader's identification with Dracula's intended victims.