Full explanation
A 6x5 grid has 30 positions, more than a typical 5x3 or 5x4 layout. For a collect-and-sweep feature, that extra real estate is the whole point.
The Mystery Reel mechanic rewards a busy board. The more Coins, Diamonds and Clovers that can sit on screen at the same time, the bigger the haul when a Pot of Gold lands and collects them all. A larger grid simply has more slots for those values to occupy, so the ceiling on a single collect is higher than it would be on a cramped layout.
The grid also pairs neatly with Mystery symbols landing across the four centre reels. With five rows deep, those reels can carry tall stacks of stickies, which feeds longer respin chains and gives the collect feature more to work with when it fires.
"Better" is doing some work in the question, so to be fair: a bigger grid is not automatically better, it is better for this style of feature. Le Zeus is built around collecting and sweeping values, and the 6x5 board is a sensible fit for that, not a gimmick.
More space, bigger sweeps
The simple version: a collect-and-sweep feature scales with how many values can share the screen, and 30 positions hold more than a 15-position 5x3 board. When a Pot of Gold collects everything at once, a fuller grid means a fatter collection.
The depth also feeds the Mystery stacks on reels 2 to 5. Five rows give those reels room to carry tall columns of sticky symbols, which supports longer respin chains and gives the collect feature more to work with. So the grid choice is not decoration, it is matched to the way this specific game pays.
The wider grid simply gives the collection mechanics more room to work, which is why a loaded board can pay so suddenly. There is nothing to optimise on your side. Just know that the big moments tend to come from the grid filling with values during a feature, not from the base game, and budget your patience accordingly.