While I was searching for the meaning of the word "libertarianism" below
The adherents of libertarianism believe that private morality is not the state's affair and that therefore activities such as drug use and prostitution, which arguably harm no one but the participants, should not be illegal.
Why is "and that" used instead of just "and" ?
One answer here is that stylistically, parallel structure helps. One approach which maintains parallel structure is to get of both thats, which is to say
The adherents of libertarianism believe private morality is not the state's affair and therefore activities...
Each "that" plays the same role of demarcating "these are the things that adherents of Libertarianism believe".
