The upshot OP is that, if you don’t want to run bespoke, possibly very detailed, long distance routes, that you have created yourself and don’t want the iWheel to control your GPS device, then your phone will work just fine *. Lots of bods do it that way, not least as it saves them the additional expense of buying (and learning) a Nav VI and / or getting to grips with its associated software, BaseCamp. Others do want to use the Nav VI fully and keep their phone for making calls, taking, saving and sharing pictures, acting as a diary, telling them the time, day and date, playing music, receiving and sending texts and / or emails, watching films, surfing the internet, engaging with UKGSer and playing games. Not least perhaps, they don’t want to worry about possible data usage, particularly abroad. Some will integrate their phone with the GPS device and the bike together. In short, it really is up to you. You’ll sell your Nav VI on eBay or on UKGSer, no problem or keep it for use on on 1600, assuming that is you are keeping the bike.
* Subject otherwise to the proviso that it won’t fail due to vibrations and / or overheat in the sun. Do either or both and you’ve lost your phone and your GPS device. So, be sure to take a map.