What is an advantage of vertically stacking horizontally polarized Yagi antennas?

When you array or stack antennas, the combined radiation pattern narrows in the plane corresponding to how they are arranged. By stacking horizontally polarized Yagi antennas vertically (one above the other on a mast), the beam is compressed in the vertical plane (elevation). Narrowing the elevation lobe concentrates more of the radiated RF energy toward the horizon, which greatly increases the effective radiated power for long-distance (DX) communications. The horizontal beamwidth (azimuth) remains roughly the same as a single Yagi. If you wanted to narrow the azimuth pattern instead, you would place the antennas side-by-side rather than vertically.