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.
are you doing the entire *arr stack as well? There are some cool tools to connect to plex as well allowing for things like requesting of shows that then integrate with he *arr stack.