What is the real purpose of Shakespeare of writing this play?

Perhaps one of the reasons to write the Henry VI plays, plus the other plays in the group of history plays concerning the Plantagenets was that the Tudors replaced the Plantagenets on the English throne.  What better way to curry favor from Queen Elizabeth than to show the previous kings to be constantly involved with wars, whether against France or civil wars within England itself.  Under the Tudors there was peace especially after the defeat of the Armada.

The part of recent English history that Shakespeare tells with the story of Henry VI is the horror of war both at home and abroad.  The peace with France forged by his father (Henry V), is broken and the Hundred Years War erupts anew.  England had no great hero once Henry V died and France's army was being lead by Joan of Arc.

Henry VI is nothing like his warrior father.  What Henry V won, his son lost.  On the home front, Henry had to fight the War of the Roses which ended only shortly before Shakespeare was born.

Shakespeare abhorred war.  He can and does glorify it to an extent in Henry V but particularly in the Henry VI plays he gives us a grim picture.

The affects of war on human beings doesn't change whether it is the War of the Roses or a contemporary war.  People die.  Soldiers as well as civilians die.  Families suffer and are torn apart by war.  Shakespeare knew this.  Just like we know men and women who fought in Viet Nam, Shakespeare knew men who fought in the War of the Roses.