It will charge more quickly. All the Plus models can draw more than 5W, (the non-plus models will only draw more than 5W when you are using it otherwise with the screen off they draw just 5W)
There is a pretty big benefit to any iPhone (newer than 5S anyways). The iPhone Plus models take forever to charge with the standard 5W charger, but roughly 30% less with the 12W charger.
Using the 5W 2.1A charger vs the included 1A I notice a difference. I didn’t know the plus could take more than 5W. Although I have to check. Don’t know if I have a charger more than 5W
oh even better, I know 2 hours is still long compared to phones with quick charge and the correct brick to take advantage but shaving 1 hour off 3 is worth the $20CAD price for the 12W