Scenes make up the book. There are short scenes and long scenes. What is their purpose? Each scene must do something. I know sometimes when I've written a scene, I'll look at it later and think,just what does this scene show? Sometimes a scene does nothing and is little more than words strung together.
So, a scene can show character development. One can write a descriptive sentence that says what the character is. She was so sweet she made my teeth ache. Now this is very descriptive and may be fine for a minor walk on character but showing this character being sweet, too sweet to another character shows the reader what this character really is.
A scene moves the plot forward. For me sometimes these are the hardest one to write since you want to move the plot forward without revealing all. The scene is used to give clues to what will happen.
A scene shows where the characters are in time and space. This has always been difficult for me. I love scenes showing the characters and how they're developing and ones who really show how the plot inches forward. One of the first rejections I received for a novel "Your characters exist in a vacuum." The editor was kind enough to take one of my paragraphs and show me what she meant. I still have to remember this.
So remember when you're writing a scene "Remember the purpose."