That’s a very fair question. They are designed to work with the charmed-kubernetes and kubernetes-core bundles. They enable storage to be provisioned dynamically to support the needs of Kubernetes clusters 
Integrator charms make use of the juju trust functionality that these charms make use of is quite new. (Juju v2.5?)
From the “Deploying Advanced Applications” page of our documentation:
Trusting an application with a credential
Some applications may require access to the backing cloud in order to fulfill their purpose (e.g. storage-related tasks). In such cases, the remote credential associated with the current model would need to be shared with the application. When the operator allows this to occur the application is said to be trusted. An application can be trusted during deployment or after deployment.
To trust the AWS integrator application during deployment:
juju deploy --trust cs:~containers/aws-integratorTo trust the application after deployment:
juju deploy cs:~containers/aws-integrator juju trust aws-integrator
Source:
https://discourse.jujucharms.com/t/deploying-applications-advanced/1061
Some more material with another worked example: