Why do some users have a Resources button at the top of their screen?

Because they're a department manager.

Specific managers have additional permissions to manage departments within admation. As a result, they'll have an extra Resources button at the top of their screens. The resources button allows you to view a summary of all tasks that people are scheduled to work on in admation for a given department. You can look at Project Resources for a project you have access to. Read more about it here.

If you would like to see Resources for a given department, you'll need to become a department manager. To become a department manager, you can follow these steps. 

You can also have a chat to your admation client success manager if you're interested in using resources for your organisation.