While it is an important international port in real-life, in the game its primary function is to provide ferry connection between continental Europe and the United Kingdom. Two ferry services are available, one to Harwich near Felixstowe and London, while the other goes to Hull near Grimsby.
Because of its convenience in-game, it's immensely popular among truckers participating in multiplayer (e.g. using the mod Truckers MP). There's even a Twitch.tv channel showing the real-time traffic condition around Europoort.