Depending on propagation, location, etc, DX happens between 2° and 10° off the horizon. So low angle radiation is what we are after for 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 signal level again).
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.