Wasted ad budget
Many paid campaigns fail because of the page people land on. Sending ad visitors to your homepage makes many of them leave without doing anything.
One page for each ad
A landing page should have one goal that matches the ad exactly. Removing the menu and other distracting links helps the visitor send a request.
The same words people search for
Using the same words in your ads and in your SEO keeps your message consistent. Visitors trust you more, and you pay less for each click.