Use of switch has authority ?

Hi there ! I’m currently developing a multiplayer game in Blueprints where players can select their character. As there is a level change between the character selection and the actual game, I’ve decided that each player would store the desired character on thier game instance (and this part works fine).
At the begin of the actual I need to bring each character from the game instances to the gamemode. As seen in a tutorial, I used ‘Switch Has Authority’ :

But recently after a friend asked me, I not sure of the use of ‘Switch Has Authority’. Moreover it seems to work fine without :

  1. Is it really necessary ?
  2. what’s its use and difference with ‘is server’ ?
  3. AM I DOING THINGS RIGHT ?

Any explication would be apreciated,
thx