I think the premise was the plex server will be running off of wifi and they didn’t think they would have wifi drivers available to them during install so they wanted to install hard wired.
No how wise it is to run a plex server off of wifi is another discussion all together but maybe for a different time.
Mostly for bandwidth and connection stability reasons… if the plex server is only serving your own home then likely not much to worry about but if you are serving plex content for ppl outside your home it can be troublesome if your wifi starts acting up and they don’t know why they suddenly start buffering.
Nothing about plex itself is limited by wifi it’s just a potentially a high bandwidth / throughput service and typically you wouldn’t want that on wifi.