Halliday & Matthiessen (1999: 239):
If the Theme of a clause (realised as the element in first position) is also explicitly located as Given, this has a very strong ‘backgrounding’ effect: the message is ‘you already know this; now use it as a stepping-off point for a further move, to something you don’t know’. By the same token, the remainder of the clause (either the whole of it or the culminating element) is strongly ‘foregrounded’. The total construction is obviously a powerful device for reasoning and argumentation.