Good landing pages mean the difference between making a sale, and a customer bouncing.
A landing page, is a page dedicated to a specific objective on your website. For many sites, this means to collect leads, for others, it means download a free resource (which is also collecting leads), or maybe selling a product right then and there.
Without optimizing your landing pages, your paid search campaigns will surely fail. It's a scientific fact.
Is it possible to see marginal success, or even great success without landing pages? On the rare occasion, yes; but in most cases, without properly optimized landing pages you will lose an incredible amount of potential business.