Well, the story does focus a lot on the character development but both sides influence each other with neither being secondary. Some things from outside will happen that force character development and sometimes the development will change the flow of the story.
It's just that the story is very long (as in 12 volumes, some of which have between 150-200 chapters) and even though I update daily (and sometimes more than one chapter), it'll take years to finish it (plus there are prequels and sequels) so I expect readers to take breaks (after all, some people like to stockpile and binge).
So even though the story does explain everything, the relationship chart is intended as a way to refresh the readers' mind after such breaks so they don't have to re-read hundreds of chapters and can just look at the chart to gauge who was who again and then get back to reading where they left off.