The reason that this is an advantage is because it allows crops to spread from one area to another very easily. This meant that an area like Eurasia with a long east-west axis could have domesticated crops spread over it quickly.
Crops can spread from east to west because the climate along an east to west line is generally similar. If you go north to south (or vice versa) the climate changes a lot. This is not true when you go east to west.
Crop diffusion is good because it allows more civilizations to emerge. One area can "invent" agriculture and their invention can spread to other places, allowing civilization to emerge. In areas with long north-south axes, each place has to "invent" agriculture on its own and fewer civilizations emerge.