Not usual in the beggining, except if there´s a paragraph elaborating what happened at the time, so when the "back then" comes, the reader "retrieve" that information.
It seems like being used for effect (letting the information sink in, sort of).
Example:http://www.theguardian.com/commentisfre ... ed-it-backIt’s true that the main arguments for joining were economic. But in parliamentary debates at that time, Tories argued for this continental commitment as a contribution to western security, against the Soviet Union.
...Back then, it was Conservatives who thought globally, while Labour tended to be more Eurosceptic and insular.
Here "back then" is also a way of wedging a parenthetical information (that is, also to make the contrast between the "global thought" on part of Conservatives vs the Eurosceptic one of the Labour.) That piece of information could have come before, but then the writer recalled the information afterwards, or then he might put it there for effect as well.