I have been trying to understand xcode..
What is a Workspace? Is there something that can't be done with a project that needs a Workspace?
Apple has a page explaining what Workspace is.
In short, it's used to group several projects together to build an app. If you use CocoaPods for example, the pods are referenced as targets from another project. A workspace helps tie your code and those pods together to make an app.
You use a workspace to share a code library amongst multiple projects.
Watch https://youtu.be/LNU6L2pnq0k and it should help you get a better idea of why/how they are used.