The events that lead up to Warlords of Draenor affect the whole storyline of World of Warcraft. Similarly, the level 90 boost affects new players, returning players, and the current players of WoW. Most importantly, the level 90 boost affects Blizzard and their sales of Warlords of Draenor expansion.
If you are a new player or you are returning back to WoW then this level 90 boost is the best chance to skip the level processes and jump right to the end game. In the same way if you have a friend and want him to play with you but you are already level 90, if he pre-orders WoD he will be able to join you at level 90. Not only that, if you have also pre-ordered WoD, then you will also have the boost and with that you can start a few level 90 toon to play with your friend.
This also means that the amount of level 90s will increases dramatically which will affect the end game. Now days the LFR is filled with new players who have gotten their level 90 toons by pre-ordering WoD. These players are quite easy to spot, as mostly their gear will not have any Gems or Enchantments. Now these players can really spoil the raid and you mood as well. I remember a Monk tank that had no gems and only his boots were enchanted get two shot at the Kor’kron Dark Shaman fight in SoO LFR. So, if you are a new player who has pre-ordered WoD and used your boost. Kindly look up the fights and try not to ruin the game for others.
If you already have a level 90 toon, the pre-order level 90 boost still affects you. You can use it to level an alt, or change realms and even factions. This helps if you do not want to move your currently level 90 character to different realm yet you are tired of playing in your own realm and want to play on a different realm. Moreover, with the awesome account bound gear of timeless isles, you can have your toon ready for raiding in a few seconds. In addition, some chests on timeless isles give Burden of Eternity when looted by a fresh level 90, which helps you get better ilvl gear.
The level 90 boost also affects Blizzard and the sales of Warlords of Draenor. Most people just brought WoD to get the level 90 boost. It is funny really; Blizzard sold level 90 boost and a promise of a new expansion for $69.99, because when the pre-order offer started WoD was not even in the Alpha. In a way, it will not really matter to Blizzard when they release WoD nor will it matter if the expansion is good or bad, as they have already sold many copies via pre-order. Well, if the WoD is bad it will result in a drop of player as we saw in Mist of Pandeira.
As far as I know, at least 75% of the players have pre-ordered WoD, just so they can have the level 90 boost. Moreover, about half of the players who pre-ordered WoD already have level 90 characters. In addition, if you have pre-ordered WoD, you are bound to wait for the game to be fully released to play what you have paid for.
so in short, the level 90 boost plays its part more then we give it credit as it not only affects the people how are playing the game but also improves the sales of the Warlords of Draenor. It increases the amount of level 90s and with it brings new players into end game contents of Mist of Pandeira. Even though some new level 90s have no idea what do it, it still makes lowers the time you have to wait for a group in LFR.