My guess is that it is a soundbite thing. With people having shorter and shorter snippets of what they say quoted on news etc it has a suggestion of being the culmination of a longer reasoned argument which the particular soundbite rounds off. It also has a hint that others may have agreed with the previous statements and that is just the end point of a view that everyone agrees with and is the logical answer.

As leeneia says " "So" means that the speaker has pulled together a number of elements which have been under discussion and is ready to provide an answer" - it's just that now you don't have to pull together the other elements to provide the answer; it just seems that you have.