Getting reputation for the new factions is going to be beneficial in different ways. Earning reputation towards the factions is going to help you unlock Pathfinder in Kul'Tiras and Zandalari. Pathfinder is nothing new so i decided to include it in here. Pathfinder will give your character increased mount speed and its account wide. Also gaining reputation will allow your character to buy gear, from a vendor, equal to Heroic dungeon quality.
Edit 20 It’s been a week since this post and a lot of people are still reaching out to me with questions which is fine. I have had a few different people reach out to me to help make YouTube videos based off this guide and more info in BFA which I’m more than willing to do. Whoever gets the videos up first I will link them here for you guys to enjoy.