These two things are important to you individually because this helps you figure out what you need in order for your game to make sense. And once you have that going along the way you can make changes, and make it more interesting. These things help because you do have to think of those things in your idea for a game. Because then everything would just fall apart, and you would probabaly have things missing in your game once you are presenting it to to others. This helps the team as well since as I said before it communicates with others in order to understand. And once they know they could add more to the game to make it more fun, or humorous whatever your theme is for the game.
The things that I found important to consider when doing a GDD is the information you are putting in the GDD. Because as you know information is important. But it's not asking for all the information about the game, like there are specfic things you put in a GDD. In the pitch it was for me well, was the process in which the game would be in. As well as the limited things you do when you have a pitch, You see for me there is a balance. And that balance is having enough information and not doing to much. I'm not good with GDDs and pitches now but hopefully I will later on this year.