Depending on propagation, location, etc, DX happens between 2° and 10° off the horizon. So a low angle radiation is what we are after a good DX. If an antenna (we don’t talk about verticals here) is lower to the ground, then the far field plot indicates a rather higher angle radiation.

For Example:
The difference between 5m and 10m antenna height is 6dB at 5° (that’s quadruple your signal level).
The difference between 10m and 20m antenna height is again 6dB at 5° (that’s quadruple your sign level).

Antenna 20m off the ground with 100W = Antenna 5m of the ground with 1.6kW.

I think you can see where this is going. Height is Might! Particularly for DX.