Functions on the services that are called after it is instantiated. These can be used to set additional options on the service, especially ones that might not be required. For instance, on the url_generator service it has specific dependencies on the non-bubbling URL Generator Service as well as a Renderer service.

url_generator:
 class: Drupal\Core\Render\MetadataBubblingUrlGenerator
 arguments: ['@url_generator.non_bubbling', '@renderer']
 calls:
 - [setContext, ['@?router.request_context']]  

If you look at the UrlGenerator class--the one that's created for the url_generator.non_bubbling service--you'll see that it creates its own RequestContext. However, if there is a current RequestContext for the Router, we'd want to use that instead of a more generic one. Since you could use this service in a context where there wasn't an active Router, you wouldn't want to make it part of the arguments array.