Depending upon which nomad group is being referred to, there may be a variety of reasons for their nomadic lifestyle. In no particular order:

Follow the animals. Some nomads depend on migrating animals for their food and clothing supply. Where the animals (like buffalo or elk) migrate, the people must follow.

Trade. Many nomadic peoples were (are) involved in trading as a means of supply. They will move from city to city or civilization to civilization with goods to barter. This will provide them with a substantial income in order to buy what they need.

Seasonal vegetation. Dry season vs. wet season. Cold vs. warm. Folks have to have water and vegetable growth to survive. Moving with weather patterns is vital for survival in order to have water and plant life. Some nomadic tribes move to avoid the very cold weather of the northern areas and find more comfort in the southern regions.

No knowledge of permanent settlement technology. Some nomadic groups simply do not have the knowledge or technology to develop agriculture, forcing them into a migratory lifestyle: see #1 and #3 above. And, believe it or not, some may actually prefer not to settle down and maintain their traditional economy: see #2.