I have an opinion on this (and it is only an opinion).
"The only reason I can think of is forcing computers in the network to trust your own self signed certificates instead of getting them digitally signed by a 3rd party CA."
If you only add one root CA then its unlikely to be viewed self signed certificate in the traditional sense, its a certificate signed by an internal CA (although that CA certificate will be self signed). Unless you whack that same cert on every box which would be a very stupid idea. You just sign the correct cert with it and then lock that CA away again never giving it network access and sneakernet signing requests when needed.
Additionally, its preferred (by me) to a wildcard certificate as a compromise of a host can only lead to interception to that one host (which it would likely do anyway) rather than everything in that domain.
Finally a third party CA is to me an unnecessary line of trust on an internal network. You can control your own internal policies but not theirs - of course if they are corrupt/compromised they can still screw you over unless you trust ONLY your own CA.So I have mixed feelings on this point.
If you protect that CA certificate sufficiently, it is no less secure than a third party, and in my view more secure than a wildcard certificate.