When you are developing something (can be a software, a plugin or as in this example a book) the simple way is having your files on your own PC, change them any time you want until you get what you wanted.
It you are developing something in collaboration you can have all the files on a share and anyone can edit, add, change anything.
It’s fine (here someone will kill me 
).
But, and it’s more true when your project grows, you’ll face this and many other situations:
- You add a great feature that breaks another feature and you want to revert
- A want to take back to a removed feature
- You and your collaborators are changing the same file
- You want to try 2 different endings for your romance
- You want to keep track of who and when changed a file in what way
- You introduce a wonderful feature but you first want to test before make it public
- You want all of your users/readers to suggest changes or fix things
A Version Control System is a software/protocol that take care of this. Is a sort of logging system/time machine/collaborative environment designed to handle with the above problems.
Git is one of those.