Peleep, I had trouble finding an answer to this one but eventually found these statements which may answer your question.
'Some cordials use citric acid and tartaric acid. These are natural acids extracted from lemons and oranges that are used to intensify the flavour of cordial.'
and this one
'What is important is to add citric and tartaric acids in some form as fruit cordials made without them seem to me to be not nearly as good.'