Maybe you want people to sign up to your newsletter, or to register for updates, or to buy a particular product or service.You set the objective, and then you design your landing page to encourage people to take that action.
Also, you can include your client testimonials on the landing page itself, that's something all new visitors want to see. Apart from the design and the content that's actually necessary, the template has been kept devoid of all designs.
A single site could use multiple landing pages, each designed for a specific type of visitor and geared towards a different outcome.
In fact, companies that increase the number of landing pages from 10 to just 15 see a 55% increase in lead generation, according to this Hub Spot report.
Or you can use specific software like Instapage or Unbounce.
For more on those, see our Envato Blog post on Creating Professional Landing Pages with Instapage or our Tuts rundown of 10 Popular Unbounce Themes Designed for Conversion.