User accounts are a way to tie your identity to a service or a set of services. By setting up an account on a website, that website can ensure that you are who you say you are (usually be checking your email and password) and give you access to interact with protected material.
User accounts allow you to access your files. They also allow you to collaborate with others, sharing the things you're working on, and allowing others to share their projects with you.
Yes. In this course, we'll be using a variety of tools provided free by Google, but in order to do so, we'll need every student to have a Google account so they can save their work and access the tools.