There's been a number of problems in World of Warcraft in the last couple of years and the addition of cross-faction gaming is a step for fixing these issues from wangyue's blog

Cross-faction content is finally coming to both the Alliance and Horde with patch 9.2.5 of World of Warcraft: WOTLK Classic WOTLK Gold. The faction divide will come in the upcoming major content update, called Eternity's End. The game's last major update, Battle for Azeroth, the news came as a shock to many of the fans. In light of the evolution of WoW's story, the inclusion of cross-faction gameplay in WoW is justifiable. In the aftermath of 8.3 the factions have joined forces to fight off adversity, and now , following the conclusion of Eternity's Endeavor players will be able to join forces with their friends in both directions for patches and expansions beyond.

There are two goals that Blizzard was looking to pursue organized instanced gaming and making this an opt-in feature. These two principles will be implemented by different systems, but they will be subject to restrictions. If it is a matter of older instances of legacy there are instances that will not accept cross-faction gatherings, and "outdoors" players from cross-factions will still remain unfriendly or even hostile in the event that the player has War Mode enabled. Players can decide whether they want to bridge the faction divide within their own gameplay, but with the introduction of cross-faction gameplay In World of Warcraft, players who opt in may have greater options regarding finding a party. Organised instances are where things become more technical.

The ability to participate in cross-faction organised instanced gaming will only be allowed for Mythic dungeons, raids, and rated PvP. Players will be able to directly invite members from the other faction to parties if they're BattleTag and Real ID friends, or members of an inter-faction WoW community. It is also possible to limit pre-made group and group finder listings only to one group if they are offering the content in which it is accessible. 

Match-made random activities, such as Heroic battles, dungeons and random battlegrounds will remain the same buy WOW WOTLK Classic Gold. Higher-level WoW content like Mythickey, raiding with groups and participating in high PvP battles with high ratings will be possible for both single-faction and cross-faction groups so it's down to the player's preference with whom they would like to have a party with.