Many people prefer using an OTG cable to connect the Spark remote to their tablet/phone. The theory being that a cable is more reliable than a WiFi connection. DJI did not design the Spark remote to be connected in this fashion but it does seem to work, depending on firmware and version of DJI Go.
Personally I have never had any issues using just WiFi between my tablet and remote. They are literally 2 inches away from each other and it's very stable for me.
Its one less wifi signal for the remote controller and mobile device's power consumption to have to deal with - I always use OTG, never had any issues - it is officially supported now.
If your WiFi connection is sketchy try the OTG cable. If everything's working fine you don't need one.
There are literally hundreds of posts about OTG on this site. Almost all of them are outdated and irrelevant since the recent DJI firmware and software updates.