Just posting these stolen images and associated blurb up here for no other reason than a) it got a landrover in the image b) it's a great visual explanation of mobile antenna and their application. c) it reminds me big is not always better.

The illustrations below best demonstrate how radiation patterns differ between antennas.

Firstly a note on gain: Antennas with 'gain' do not somehow magically create the extra power but simply focus the radiated RF into narrower patterns such that there appears to be more power coming from the antenna in the required direction. So in essence, the design robs power from other directions and superimposes it on the radiation in the intended direction. But if the increased gain antenna doesn't suit the required application then the gain can also be a loss. As in the diagram above with the two landrovers, the 6dB gain antenna design has a further radiated signal but it's in the wrong place for that application and therefore the 3dB antenna would be a much better performer.

The diagram above shows the radiating pattern of different gain antennas. As the electrical design of the antenna is modified to increase the gain, the omnidirectional pattern is squashed in a vertical plane, therefore furthering the signal's coverage. A high gain antenna will therefore give increased coverage on flat terrain but the elevation will be limited making it unsuitable in mountainous country.

Hence the antenna choice for undulating and mountainous terrain is different than flat outback conditions.

The lower dB gain antennas have a wide radiation pattern which is ideal for the traveller in undulating terrain. The wider radiation pattern allows the RF to travel up and out of the lower areas.

The higher dB gain antennas have a flatter and longer radiation pattern which is ideal for flat or slightly undulating terrain. 

The very high gain antenna are more suited for hill top DX or flat desert use.

(reference LINK)