You're probably trying to promote something with your website, like a product, service or idea. The first step in getting your visitors to act is to begin a dialog with them. They see your site, and send you an email. Then you have their name and email address, so you can follow up in a more personal manner. When your visitor clicks on the link, an email form is opened and the visitor can send you a regular email.
Email has some problems.
Email is boring. Nobody likes to type in email messages. It's not really a part of your web page. The email form opens up into a big blank page. It's hard for your visitor to get started.