Many of WGT's games are set up like tournaments. Also note that WGT players have a scoring average instead of a handicap.
The rules of golf have a provision that lets a tournament committee invoke a local condition of the competition requiring all players to use the same make/model of ball for an entire round. it is known as the One Ball Condition. Breach of the rule results in a player being disqualified from a tournament that has invoked the local condition.
Rather than disqualify a player who did not make sure they had a sufficient stock of the same make/model of balls to complete a round, WGT provides a free Starter ball.
WGT has a pop-up message that warns players when they start using the last of the make/model ball they are playing, and gives the player the opportunity to buy more of the same make/model of ball.
You can read about the One Ball Condition in the USGA rule book in Appendix I - Local Rules; Conditions of the Competition, Part C, 1., c.