Find usages can be used to find all the pieces of code referencing a specific element, such as a class, method, symbol, or many of the others. RubyMine can also find dynamic usages, including Rails associations, factories, or delegates. You have the ability to specify the scope and search through an open file, project, referenced libraries, and so on.
Find usages is very helpful for investigating a project and refactoring your code. For example, the Rename refactoring requires high accuracy when finding usages for a target element, so that you can trust the IDE when making such changes.