Wardens: I'd imagine the wardens are guardians of the druids' barrows; and double as prison guards when the need arrises. The barrows are powerful places; there to amplify or contain immense magical energies - hence the barrows and their guardians would be a good choice to keep the dangerously powerful NE villainy (ie., Illidan) under lock and key.
Or then again, for all we know the Wardens were formed simply as a response to Illidan needing incarceration - he might have been their only prisoner.
Wardens also capture criminals and carry out executions. Their jurisdictions extends beyond the barrows, they're more than just guards.