I assume you are quoting the rules for the MLS Next Flex event in Maryland in May. They need one winner of each bracket to qualify for playoffs in Dallas the next month. Ties complicate that where there are only three games for each team. They could have just done the typical tie-breaking procedure to determine the bracket winner, but there would have been arguments about whether goal differential encourages running up the score or goals against encourages packing it in, etc. Instead, they decided to do PKs, which at least provides the kids and GKs something more fun and exciting than tie-breaking rules and it doesn't take up much time. You still get more points for a regular time win than a shootout win, so there's no incentive to hold out for a tie. These rules are now being used in all games in MLS Next Pro as well.