In-game Combat Missions are coded months in advance so they can be tested on the PTS before coming to Live.
Sometimes, there are additional ideas for contests, giveaways, or engagement beats that are created after a patch is "locked in". These things are like the Chips-for-Treats campaigns, or recruitment campaigns, or whatever we dream up and think is cool. They are always optional and players can choose to ignore them, so you shouldn't feel pressured to "have" to participate. In the end, it's more activity for those that would like more things to work on.