Why is the development of written history important?

Well the Greeks wrote about fables, myths, and legends. All included some of the Greek gods. When written history came from Thucydides he knew that he was going to write without gods. He separated fact from legend.
Written history is very important to know how we got to where we are now. The traditions of oral history have well served societies but there are so many diverse people on earth and so many centuries of history to pass on that in must be written so that as many people as possible have access to it.

The importance lies in the fact that if you don't know history, you will always be guessing how to deal with situations that are occurring now if you don't know what the outcome may be when it was dealt with in the past. Unfortunately, too many world leaders don't seem to be aware of this. There are well known sayings that tell it more simply; "What is past is prologue" and "Those who don't learn from history are doomed to repeat it" (please don't tell me it's a misquote, I know it is but it's my preferred misquote).

Written History is so important because it lets the reader know what really happened or at least the "Victor" thought what happened. History is written by the victor". for example if the Nazis won WWII then 50 years he would be known as "The Great Leader", 100 years he would be known to as like a God, in 1000 years he would be known as "A god with great leading abilities who defended monsters and demons from his homeland". Obviously that is very different then what really happened and what we know today.

Oral history can have many misconceptions the story would get greater and greater like i mentioned before and so it is unclear of what really happened