Maybe you remember : a few days after the Marvel skins were released on the BR, STW received a ton of VBucks mission alerts.
The VBucks rain went on while some new Marvel skins were released.
After a while, the rain stopped and we came back to very casual VBucks mission.
Two weeks ago, the Terminator skins were released. Not as good or famous than the Marvel ones, but still pretty famous enough to concentrate a bunch of purchases by the players (BR or STW).
After that, we had nearly ten days of VBucks missions in STW.
If you ask what is the point doing so, any company needs to prove its business is attractive (to partners, shareholders, etc) by showing some clues and indicators.
For a cash machine as the BR is (was ?), Epic needs to show that the potential VBucks to be spent by the players is regular (maybe an indicator of the average VBucks active players have as a stock market potential).
When players spend much VBucks on BR, we are given VBucks so the curve is flatten.