Get large enough (you will have moved beyond fully insured plans to self ensuring your population long ago) and you'll continue to look at opportunities further down the supply chain in actual healthcare delivery itself to better manage, forecast, and optimize costs.
Primary care and Rx are usually the first place to go.
My guess is this is as much about that dynamic, as it is about buying some good leadership (look at the board/leadership of One Medical and you'll see many familiar faces if you are in heathcare/plan/PBM space).
Will be very interested to see if they keep this mainly in house, or if they begin to scale beyond that.