The answer is: to inform the reader of the reasons for the differences between editions.
The author is presenting here a brief and condensed history of the tales written by the Grimm brothers and making it known to the reader that the reasons for their work having as much as seven editions is that throughout their lifetimes, the Grimm brothers strived to achieve a product of a higher literary quality that could resonate more vividly amongst their literary public, which they seem to have accomplished successfully since their stories were only second in popularity to the Bible in Germany.