v.123 - Tempest: Into the Storm Update Notes

For just one quest at a time, using the arrows and enter is no big deal.

However, 'y' was very useful when you need to repeat the quest a lot. For example, the "girl with a crush" room at Red Leaf High: there is no "right answer". No matter what you click, the RNG decides whether you pass or fail. If you fail you have to do it again. Holding down the 'y' key would let you very quickly redo the quest until your answer (always the first option) is accepted.
Same thing happened with the Rock-Paper-Scissors event we used to have. Fastest way to win it was to hold down 'y' until the first option came out winning.
Another example is trying to get into a hotly contested map, such as a non-instanced boss or a popular PQ (yeah, right). Again, holding down the 'y' key meant spamming entry attempts until the NPC let you in.

Spamming the NPC button does the same thing. In the example of the "girl with a crush" quest, all I did was hold down the NPC button until it was right.

I've never once used the 'y' button for anything. Just using NPC chat worked just as well if you were going for an accept/yes every time.