Why such a huge amount of bonus XP for a max level mission you wonder? Why, it is so you can upgrade your followers. Once they reach level 100, any additional XP an uncommon or rare follower earns goes towards upgrading them. Thus a 'green' follower can become 'blue', and a 'blue' follower can become 'purple' (epic).
Each upgrade will give them additional traits making them more versatile.
'Drove the Ruiner'? GJ Blizz
Garrison Timer Mobile App
[Upon completion your 随从 will gain: ]
Guide to the Garrison Barracks
Missions and Followers math for maximising success chance
Warlords of Draenor Garrison Missions Guide