Early email systems were proprietary: private networks within companies or user networks which could not communicate with other systems. As the Internet developed, these email systems were modified so that you could send text messages to anyone with a similar system. Sending files or viewing HTML mail was (and often still is) problematic because these programs have proprietary mail formats, which limit their ability to communicate with other mail systems. As the Internet became a stronger force, new email standards were created to keep pace. Post Office Protocol, or POP for short, is an example; POP programs share a standard way of communicating, which means you can exchange mail (including attachments) easily with anyone on the Internet from their own computer (but not others). WebMail is the latest advance in electronic mail, allowing users to send and receive mail simply by using any web browser from any computer anywhere in the world.
mail programs are designed primarily for single computer use. Once you set up your email account by entering your POP3 and SMTP server names, you're stuck using that individual computer to send and receive mail.