It is, as tchrist states, a question tag.They are described as either 'variant tags', where the tag's form changes to match the subject of the sentence ("he's crazy, isn't he?", "they did it, didn't they?"). This is the most common way to construct a question tag in 'standard' English. Or as invariant tags, which means its form does not change to match the subject or the sentence, like 'so'. Other common invariant tags are:
'like' -which is a more common Irish form than 'so', from my experience.
Invariant tags are 'all purpose tags' and common features of second language learners, who often use these forms as they may match the tags in their first language. So, 'isn't it' may be applied universally, in order to bypass the need to learn the variant forms in 'standard' English -call it an expedient or efficient communication strategy.
Question tags in other languages often are invariant, e.g. 'oder' in German, 'n'est-ce pas?' in French (as in tchrist's post above). Indian English has an invariant tag 'isn't it?' and this is also a feature of some Welsh speakers (although I could be thinking of this Pot Noodle advert with welsh voice over http://www.youtube.com/watch?v=JrNuT9sn0Gc -and it can't be viewed as invariant in this case.
They may appear as unnecessary ('unneeded') and in some senses they are. Linguists would term the OP's 'so' as a discourse marker and of little practical meaning, syntax independent and making no change to the meaning of the main sentence.
However, tags can express a 'commenting or questioning function' (McArthur 1992) or as 'phatic communion' -a device in language to demonstrate feelings or sociability -'social oil' in the art of conversation to facilitate inclusion and intimacy. In female speech their use is characterised as a help in the co-construction of dialogue (Cameron D, 2001 Working With Spoken Discourse). On this theme, Tannen (1990) claims that female subculture uses language to build equal relationships, while male subculture uses language to build hierarchical relationships. Tag questions are a observed feature of female spoken discourse (see Lackoff ,1975, as well).
If anyone is interested there is a tonne (metric) of research available investigating question tags from many angles. Google search and an Athens account will yield many papers and articles comparing use, meaning and significance.