Workspaces are optional and can be used to create separate spaces in your learning site.
For most organisations, one workspace will be enough, you can do everything you need to in a single workspace to achieve your training needs. Within an organisation you have Organisation Units and Groups to further organise your learners.
Multiple workspaces are important if, for example, you have several organisations within one training site. Workspaces will allow you to create a separate space for each organisation, with its own organisational hierarchy, administrators and users.
An administrator can be added across multiple workspaces if required.
You might use multiple workspaces across a single organisation for reporting, searching for users and adding courses. These are actioned at a workspace level, so you can't, for instance, run a report across all of your workspaces, you need to dive into each workspace individually and run the appropriate report. This is the same for searching for users and creating courses. You can only access one workspace at a time.