Why is Dracula written in its given form?

 Dracula's form varies from letters to journal entries, both forms that one would share private and intimate subjects in. Stoker may have constructed his novel this way as to allow the text to carry substance and for the reader to trust it as one would not lie in these private forms. This may be so as to allow the supernatural events that follow to be more believable to the reader and also allow for a sense of 'suspension of disbelief' as the apparent appearance of truth allows the reader to engage sufficiently with the text. 

The privacy in these letters is obvious; 'This is a secret'. However as the reader is allowed to see them, it creates a feeling of intrusion of privacy, a theme that is omnipresent throughout Dracula.