The transcoding isn't just about bandwidth, it's about compatibility. I store most media in H.265 because that's the most space efficient, and most things can play it, but a decent number of devices still need H.264. I also have older content with DTS soundtracks, which isn't supported by a bunch of new devices.
Plex/Emby/Jellyfin will handle that compatibility by converting on the fly.
Plus, when using a Fire Stick, or Google TV Streamer, or old Chromecast it just isn't easy/possible to browse a SMB share.