Standardizing aspects of a REST API is crucial for making it developer-friendly, as inconsistencies can lead to bugs, code debt, and maintenance issues. Guiding principles include using specific HTTP methods, standardized variable names, consistent responses, error reporting, and idempotency. Standardization helps ensure predictable behavior, reduces the need for special cases, and makes the API easier to maintain and expand in the future. By following these best practices, developers can create APIs that are more user-friendly and efficient.