Why Should I Use Landing Pages?

The short answer is because they help increase your conversion rates and lower your cost of acquiring a lead or sale.
The main reason for this is that targeted promotion or product specific landing pages are focused on a single objective that matches the intent of the ad that your visitors clicked on to reach your page.
If you consider the example of sending traffic to your homepage vs. a standalone landing page, you can understand that your homepage is designed with a more general purpose in mind. It speaks more to your overall brand and corporate values and is typically loaded with links and navigation to other areas of your site.
Every link on your page that doesn’t represent your conversion goal is a distraction that will dilute your message and reduce your conversion rate.
A great example of this can be seen on the Campaign Monitor landing page.

Message Match

A fundamental aspect of conversion centered design is message match, which is the ability of your landing page to accurately reflect the messaging presented on the upstream ad. Most visitors are very impatient and will leave your site within a few seconds of arrival if you don’t reinforce their mission with a matching headline and purpose (quickly and clearly).
This example shows a comparison of good and bad message match on a landing page for a PPC campaign:
An example of bad message match
Ad: Get 50% off a Dell Inspiron 9000
Landing page message: Welcome to Dave’s Computer Store
An example of good message match
Ad: Get 50% off a Dell Inspiron 9000
Landing page message: Get 50% off a Dell Inspiron 9000 at Dave’s Computer Store
By ensuring a strong message match, you are letting the visitor know that they made a “good click”.