National unity is important because it arises when a country is in crisis, and government parties who are normally in opposition to each other decide to work together to solve pressing issues. The parties decide to put aside their differences and work together because they are all bonded by a love for their country.
When a country is in times of war or severe economic crisis, sometimes a national unity government is formed. National unity governments are founded on the basis that if opposing parties do not start to work together, the country could either lose the war or fall into even worse depression. The parties are united by their love for the country, and this unity often helps a desperate country make important decisions.