Suramar questline throughout the current Legion patch. As we are known, Nightfallen reputation, Pathfinder unlock, Mythic dungeons unlock all demand the completion of Suramar questline.
Suramar map opens when to get to lvl110, and it includes eleven chapters in which the total quests are more than the summation of quests in the other two maps. The player can start the initial task of Suramar after getting to lvl110, and discover the flight path from Dalaran to Suramar.
Not same as other main/side missions that need NPC to guide, all of the side missions in Suramar can be started only when you run to the task place, the completion of Nightfallen reputation in different stages can start the Suramar quest as well. The last two Mythic dungeons Court of Stars and The Arcway will be unlocked When to get Honored(8000/12000) reputation. But to finish the last Suramar questline, need Nightfallen reputation exalted and Normal Xavius died. The Suramar story continues in Patch 7.1 with a nine-week quest series: Suramar Campaign Quests, culminating in the Insurrection achievement and the Arcanist's Manasaber mount. 
Ancient Mana is the most important to Suramar quest, as a foundation for any quests. Through some special questline,  you can increase the upper limit of Ancient Mana, lighten all Mana Nets also will improve the speed of Ancient Mana acquiring.
Of course, the reward of completing Suramar mission is very rich, such as a mass of Artifact Power, WoW Gold, Order Resources, Blood of Sageras. Also,  it upgrades pretty much of average item level.