Answer:
The Arabian Peninsula is largely arid desert. There are a few oasis areas but just not enough fertile farmlands are available to allow for a agriculture based way of sustenance.
Without a farm to raise food, people use animals a main food source. Feeding animals requires grazing. The rainfall patterns on the Arabian Peninsula dictate the pattern of nomadic life.
(This is very similar to the way that the indigenous peoples of the America's followed the buffalo.)
The oasis areas,where people have settled to farm, are where the nomads trade animal and goods for grain, vegetables or other agricultural goods.
Time and season dictate the pattern the nomads will follow each year.