Can someone explain why you need to bring 4 of each card to a tournament when three of each are used?

On page 4 of the Tournament Rules it reads: “In addition, players must bring 12 unique objective, deployment, and condition battle cards, four of each type. When an army list is required, players should bring a completed list or arrive at the venue early to fill one out.”

On page 7 of the same rules it reads: “Players shuffle the blue player’s objective, deployment, and condition decks separately, then draw and reveal three cards from each deck, laying out each category in a horizontal row oriented right side up according to the blue player’s side of the battlefield.”

There isn’t such a rule that I have seen in the RRG or the Learn to Play.

Am I missing something?

you Bring 4 of each, even though you may have more than that, given the vapourator expansion and potentially more to come.

It gives you limited control, without letting you stack EVERYTHING in your favour.

Including 4 in your deck when only 3 will be used makes the scenario just a little less predictable. Blue player can't necessarily rely on getting a specific card.

As you pointed out, this rule only exists in the tournament regulations. Outside of that, you play it however you and your opponent agree. You could follow the tournament rules, or shuffle your entire collection and draw from there, etc.