Symbolism, Imagery, Allegory
Check the locks on your windows tonight: You don't want foreigners invadi... um, we mean vampires entering your home.
If Dracula's immigration to Britain can be read as an allegory about foreigners invading England, it seems reasonable that border-crossing in general will be important in Dracula. Individual homes are like mini countries for Dracula to invade, so Stoker spends a lot of time describing Dracula's entrance into various homes.
The vampire is unable to enter a house where he hasn't been invited, which is why he spends so much time in the form of a bat hovering around Lucy's window, and why he entices her outside while she's sleepwalking so that he can drink her blood out of doors. It isn't until he's gotten a wolf from the zoo to break through her window that he's able to enter her home and drink her blood in the comfort of her own bedroom.