Every topic you study or book you read contains more detail than you can recall all at once. We don’t ever really need to do this as our brains connect ideas into schema allowing us to create a summary map of the knowledge that guides us to the details when we need them. If you condense a large body of knowledge to some big ideas or key stages in a process or narrative, you can add details back in, making the process of studying a large topic possible. Summarising also helps you to understand the material more thoroughly.