One important thing to note about Stoker's Dracula is his use of verisimilitude, which is the quality of something that gives the appearance of truth or reality. Stoker was trying to make his story sound as real as possible for his readers, which is why he used diary entries, letters, and newspaper reports to keep his plot advancing, rather than only using narrative. This is also known as an epistolary novel (see link below). The log of the Demeter contributes to this as it gives very "real" details about the ship's journey.
We also get some very creepy details about the journey...the storms, the loss of the entire crew...details which contribute to the mood of the Gothic novel - "Gothic novels generally focus on mystery and horror, and they usually have some supernatural elements." Again, please check out the link below for more information on Stoker's style. Good luck!