Why do I need a DVR Tuner like a HDHomerun to use the Plex DVR service?

If my Samsung Smart TV has a digital tuner built in, why would I need at add an HDHomerun box to the configuration?

Thanks in advance for the feedback

Because Plex Media Server is not installed on the Smart TV, only a Plex App is installed perhaps on the TV which connects to your Plex Server. Your Plex Media Server can not see the tuner built into the TV.

Right now I run my Plex Media Server on a Mac Mini which is connected to the TV. All my content is stored on a local WD My Cloud.

No matter what, one would need a DVR Tuner to use the Plex DVR functionality?

Yes you need a tuner that the plex media server can reach. Tvs do not make their tuners accessible to anything other than tv

See: https://www.plex.tv/features/live-tv-dvr/