TL;DR: Server side transcoding of OTA HD recordings for playback over wifi. And "channels"
I have the plex server on my main media server, which also records OTA TV via an antenna. I often want to watch recordings on a tablet (older kindle fire) which is just on the edge of being able to playback 1080i (16Mb/s) content if everything is just exactly right with wifi. In my old house, with plaster walls, that was seldom the case. So I used the plex server-side software to transcode the video down to 3Mb/s and was able to reliably watch my recordings on my tablet from the other side of the house. These days, in a newer house with drywall, I'm finding less need to use PlexBMC, as the Kodi NFS access of the files works reasonably well.
The other thing that the plex server gives you is access to Channels (1) which don't exactly line up with Kodi's plugins. Even if they both support the same thing (say PBS or NFL Gamepass), there will be times when the Plex channel works, but the Kodi plugin does not. So it is good to have both. And, oddly, this is where the Plex Android client really falls down here. I've often had times when the Plex client simply refused to play channel content, but the PlexBMC Kodi plugin worked flawlessly.
(1) Channels are what the Plex folks call 3rd party plugins to provide access to on-line content, like PBS, NFL Gamepass, etc.
My understanding is that Plex has centralized media servers with one or many thin client where as Kodi is an all-in-one solution. You can run Plex server and client on the same box as well.
I have the plex server on my main media server, which also records OTA TV via an antenna. I often want to watch recordings on a tablet (older kindle fire) which is just on the edge of being able to playback 1080i (16Mb/s) content if everything is just exactly right with wifi. In my old house, with plaster walls, that was seldom the case. So I used the plex server-side software to transcode the video down to 3Mb/s and was able to reliably watch my recordings on my tablet from the other side of the house. These days, in a newer house with drywall, I'm finding less need to use PlexBMC, as the Kodi NFS access of the files works reasonably well.
The other thing that the plex server gives you is access to Channels (1) which don't exactly line up with Kodi's plugins. Even if they both support the same thing (say PBS or NFL Gamepass), there will be times when the Plex channel works, but the Kodi plugin does not. So it is good to have both. And, oddly, this is where the Plex Android client really falls down here. I've often had times when the Plex client simply refused to play channel content, but the PlexBMC Kodi plugin worked flawlessly.
(1) Channels are what the Plex folks call 3rd party plugins to provide access to on-line content, like PBS, NFL Gamepass, etc.