The objective of every business is to be profitable and that means leads to potential customers that convert to sales. Landing pages play a key role in achieving this. Businesses advertising online need to direct their traffic to landing pages through PPC, sponsored links or display ads. Now the landing page will play its role. That would be generating sales, sign ups or even subscribing that depends on the objective of every business.
A landing page is where you want to direct leads to and its important that you have it before you start adverting.