For #1, context gives a little background information about that passage or sliver of text that you are reading. If you dive into chapter 6 of a 20 chapter story then you will be confused because you don't have the context to decipher the text.
For #2, a key scene can give you insight to a lot of things happening. It can explain a back story, introduce characters, build the plot and so many more things. It just depends on the direction the author wants to take.