Bellular Gaming did a video about the upcoming, and imminent, in-game event that will precede the launch of World of Warcraft: Legion. This event will come with the pre-expansion patch to the game that will bring all of the new and revised game mechanics, content, etc. into the game (and then lock most of it behind the paywall that Legion's purchase actually is).
The takeaway from this video is that, if you were intrigued enough by Legion to buy into the game or return to the game, this is when you should hand over the sheckles and play. In addition to the gear, which is reason enough for those who otherwise would be farming Tanaan Jungle for a few more weeks, this is the time because it gives you a few weeks to get acquainted with the new state of the game and decide which character you want to take first into Legion's new content in the Broken Isles. Your fellow players will thank you for knowing how to play your character competently, so seize this opportunity to git gud.
Expect this testing on the Beta realms to do much of the heavy lifting, and then get on the Public Test Realm for a bigger test to ensure that stuff works as intended under the huge stress of player populations before finally going live. If you have multiple character at the current cap, then this will keep you busy until Demon Hunter Early Access hits (assuming that you bought in). Follow channels like Bellular's if you want more good stuff like this.