I rely upon FastMail for:

• IMAP: Keeping your email on the server is so much better than downloading it. When a new email virus flares up, you don’t suffer through interminable downloads of trash. And email access from multiple clients just works.
• Security: I access my email via WiFi. Fastmail lets me use SSL for IMAP and https for webmail. There’s not much to see in my email. But I feel better knowing that only I know for sure.
• Subdomain Addressing: If you have the paid email account me@example.com, then anything@me.example.com is also a valid email address. And if anything happens to match a folder name, then incoming mail will automatically be stored in that folder.

FastMail is central to my email process. I get email as part of my hosting service at TextDrive. But multiple email accounts are a pain. So most of my email addresses forward to FastMail and subdomain addressing puts it in the right folder.