There may be circumstances where OTG may improve the range, (as well as improving the video signal).
In the UK, for example, European Android phones don't have the 5GHz channels that the Spark tries to use for the link to the phone.
This means that the link then defaults to the 2.4GHz channels and the Spark then uses 5GHz for the link from aircraft to controller which may have a potentially shorter range.
If you use an OTG cable, you're then able to select either a 2.4GHz channel or (if they're busy) a 5GHz channel for the aircraft-controller link.
This isn't a problem if your country has the full 5GHz channel spectrum available in your phone, as the Spark chooses one of those channels by default for the phone-controller link by default, so you won't have noticed it.
I struggled with this when I first set up my Spark with a UK Android phone.
Correct me if I'm wrong, by all means, always happy to learn.
EDIT: Here's a post from another UK owner to confirm this:
Bumping this as I've been experimenting with ActiveTrack recently and I have to reset the Spark's WiFi EACH TIME I want to connect with my phone, the reason likely being that I'm in Europe and that the Spark defaults upon booting up to an illegal 5.8Ghz frequency that my phone (an Android) cannot connect to. Resetting WiFi apparently forces 2.4Ghz, at least temporarily.
Hope that helps someone !