Some are designed exclusively for Mac only, some are cross-platform that provide their services for Mac.
There are tens options of email clients for Mac apart from Mail. But, we are not going to discuss about it. Mac is basically has its own default desktop email client app, Mail. Some also equipped with additional features to manage tasks, schedules, contacts and a lot more. The common reason why people prefer to use desktop email client is that they mostly are capable to handle multiple email accounts. A desktop email client app make it easy for organizing inbox. Some Mac users prefer to access their email inbox from a desktop app rather than web browser.
