Having reached the practical limit of what can be done with increasing the clock speed (the "Megahertz Race"), manufacturers are looking for other ways to speed up processing. One way is to squeeze multiple CPUs onto a single chip, and let the Operating System use them to multi-task.
If you are not doing Video processing or top-end gaming, you don't need a quad-core.
If you are just doing surfing/e-mail/office tasks, you don't need a quad-core, and probably don't need a dual-core.
Hope this helps.