My question is would this be helpful to run an directory authority?
If some random person could become a Directory authority, that person would be able to pull off some serious attacks on the whole Tor network. Currently, there are 10 Directory Authorities which are hard-coded in Tor's code. They decide which Tor nodes are good and which are bad, which versions of Tor are recommended and so on.
But if you want to create your own private tor network (For example, anonymously access your computers at your private network that can't be accessed by regular Dir Authorities), tor nodes in your network can use a specific Directory Authority using
Quoting Tor Manual:
DirAuthority [nickname] [flags] ipv4address:port fingerprint
Use a nonstandard authoritative directory server at the provided address and port, with the specified key fingerprint. This option can be repeated many times, for multiple authoritative directory servers. Flags are separated by spaces, and determine what kind of an authority this directory is. By default, an authority is not authoritative for any directory style or version unless an appropriate flag is given. Tor will use this authority as a bridge authoritative directory if the "bridge" flag is set. If a flag
orport=port is given, Tor will use the given port when opening encrypted tunnels to the dirserver. If a flag
weight=num is given, then the directory server is chosen randomly with probability proportional to that weight (default 1.0). If a flag
v3ident=fp is given, the dirserver is a v3 directory authority whose v3 long-term signing key has the fingerprint fp. Lastly, if an
ipv6=[ipv6address]:orport flag is present, then the directory authority is listening for IPv6 connections on the indicated IPv6 address and OR Port.