List segmentation just makes direct marketing more efficient and effective. By knowing the demographics of the potential customers on a list, marketing and advertising can be tailored to better ensure that the intended target is hit. An example is identifying married females with children. These segment members would be interested in toys, educational materials, home and garden items, etc. They may be able to pay more than younger customers; but are still interested in value, payment plans, and free trial offers. Marketing for this segment should be straightforward, clear, and to the point.
Hope this helps!