I don't have a data plan and I don't have cellular. I bought my LG Stylo 3 to use as a dedicated display/control platform for my drones. I temporarily connect to my home network to get maps and mission information from VLM, otherwise, it has no contact with the outside world.
I believe Litchi has a cleaner interface and presents the information in a more usable manner. I can turn things on or off easily. This thread was supposed to be about turning off bottom sensors and Litchi can easily do that. Litchi supports inexpensive ($10 at WalMart) VR goggles. It has about twice as many intelligent flight modes as DGI GO and I believe they are presented in a better manner. It has HomeLock, which allows you to fly straight toward your Home position without changing altitude. That is pretty useful if you loose sight of your drone and would like to bring it back into view, but you aren't sure what direction to fly to make that happen.
I got a Windows program (VLM) that lets me create missions on my laptop, and then send them to Litchi for flying the waypoint route. It also lets you send it to Google Earth so you can preview the route before you fly it. It is pretty amazing. Here is the link to the info on it:
Virtual Litchi Mission
If I have a problem with Litchi, I can send an e-Mail to their tech support and I will typically have an answer back from a real live person within a couple of hours. Try that with DJI GO.
I feel like I sound like a salesman or shill for Litchi, but that is not the case. I just really like it that much and want to share the news.
So,,, to answer your original questions. "Any benefits to using Litchi without a data plan? Or cellular?" Yes, there are some BIG benefits.