The plot is the foundation of every story and the origin of every plot is an idea. Just a few words could already be an idea, or a game. J. K. Rowling for example came up with the idea for a scene in Harry Potter while playing chess and developed the entire story around this. A song could also bring up some ideas or what some hobby-writers do is give each other a set of three words and then they try to create an idea with those words.
Even though you can get far with an idea itself, most writers really need a plot to write an entire story, some even need a step by step manual for each chapter. However, it is not impossible to start to write with just an idea, as long as you know where you want to end or what the story is about. For example, if your idea is to write a story about eight clans from which three form a pact and attack the others, the end may be that the three clans will be wiped out. And the main course of the story is about the fighting of the clans. Create some characters and you're good to go.
Despite this sounding really easy, it is not. What happens most of the time is that the writer's inspiration runs out and the story ends without it being finished. This is why a plot is important. It gives your novel structure. If we take the same example about the eight clans, this could be a plot written in a small story:
"Eight clans have lived peacefully together for many years, but when three clans form a pact and betray the others, a war is sure to come. Altair, the leader of the Dragon Clan, Goku, the leader of the Lightning clan, and Ash, the daughter of the Rose Clan leader, are trying to come up with a plan for the war. After this plan has finally been made, they're travelling across the entire planet to find the two remaining clan leaders and all the people who belong to the five betrayed clans. They want to create an army big enough and strong enough to stand against the Wolf, the Onyx and the Crimson Clan. The biggest problem they're facing now is how to find all the clan members without revealing the existence of non-humans on the planet Earth. Altair, Goku and Ash split up and search the entire world, but when Altair and Ash have managed to gather quite a large army, Goku is nowhere to be found and it appears the Wolf Clan has kidnapped him. Altair and Ash set out on a journey to find Goku and while they do this, the battle of the Eight Clans is about to begin. When Goku has been saved, the three join the battle and they defeat the leaders of the enemy clans. After the fight, all clans have suffered losses, but they're all really grateful to Altair, Goku and Ash. A new peace treaty is made and the Rose Clan even makes Ash their leader. Together with Goku and Altair, she becomes the Guardian of the Eight Clans who now live peacefully together again."
This is just an example, but what's important is that you see the first event leading to another and so on. All events here are the main events that happen and that the writer will definitely follow. In each plot you have the introduction in which characters and the main problem are introduced, followed by the building up of the story. This is what leads to the climax, which is the third part of a plot. This could be a fight, the confrontation between enemies or a big discussion. After this, the fourth part comes, which is known as fallen action, or just clearing things up after the climax. This could be explaining why something happened or deciding to make another peace treaty. The last part of a story is the conclusion, which is basically if the story ends well or not. The first reason why a plot is important is for the writer. When you follow your plot, there's no way your story could suddenly turn into something else.
Then there are several reasons why readers could use a plot. First of all, it is an introduction to the characters and their roles in the story. Because of this, the reader creates affection for the characters he likes and he is motivated to read on, because he wants to know what happens to his favourite character. Because of the plot, a reader can continue reading in a pleasant way with order in the structure. This is the effect of a good plot, because every event leads up to the climax of the book, but by gradually releasing information and by slowly building up the impact of the events, the reader doesn't want to put the book down. He wants to read on, he wants to know what happens when the main event, the centre of the novel, is there and what happens when it's over.
A good plot is what is remembered. If your plot is not clear and unstructured, the memory of the book won't stay or will be more vague than when a plot is really well-structured.