As a rule of thumb for any document you're working on, you always want to have an original or earlier version of the file to compare your current version to.
When working on a file, especially during collaborative work, it's easy to lose track of what changes have been made. A reality which, as you move further and further away from your original version, can lead to a loss of focus and direction.
If you're working in SharePoint, there's no need for you to do this manually. The platform has a 'Version History' feature where you can access previous versions of the file.
The great thing about this feature is that it saves not one but multiple versions of your file in varying states. Meaning you are given a timeline of the evolution of the file, helping you fully assess the impact of the changes that have been made.
Here's how to use it.