In a large corporate environment, i.e. multi-site or separate application devlopment, it may be advantageous to proxy internal repositories.
To do so, add the a new mirror stanza (one for each repository to be mirrored) and set the mirrorOf section to the same name as the repository being mirrored, e.g. central, snapshots, inhouse, inhouse_snapshot, external_non_free, external_free
If you have a multi-site environment you will need to configure your proxy so that it retrieve the artifacts over the network rather than from a local file system. You should have enough knowledge to be able to do this yourself. (Author's note: I don't have such an environment, so if someone actually does configure their environment like this and run's into gotcha's, please update this document)
Generally if your environment is not large and you have direct web access to the internal repositories then there is no benefit from choosing to mirroring internal repositories.