Sometimes, an organization might not need to update the puppet classes, or use them at all. In that case, import:puppet_classes is unnecessary and in a large installation can take up to 40 minutes to complete. If the task runs once an hour that means a wait of 1 hour and 39 minutes for an unlucky user to get his environment in foreman.

This task only imports the environments so it is much faster and can run very often to keep the environment list updated. In our case it runs every 5 minutes.

https://github.com/theforeman/foreman/pull/541#issuecomment-16915521