Too many games of UNO end in very predictable and unspectacular fashion because one player is clearly ahead but their neighboring players don’t have any cards to throw down.
Allowing every player to attack any other player not only mitigates this issue but also makes the game much more dynamic.
Playing a +2? Simply point at the player you want to draw two cards. With a +4 you could even let the attacker split the attack and say “you take 2 and you take 2”