The ability to get a single feed of what they are doing that they can easily send out to everyone without having to create a mailing list.
Being able to easily react to any status that your friends post, and being able to comment on that. The email equivalent would be a separate email thread for each status update from each friend. That would quickly get messy.
And a big one for me is being able to create an event, and invite friends to it. Any updates to said event can then be communicated to all those interested without you having to make updates to a list of email addresses.
Maybe you can handle your social life fine but some of us who are more incompetent at it like the assistance of some automation. I'm only partly joking.
Those sound like tools for very immediate, personal attention. Facebook allows for that (like messaging or writing on someone else's timeline directly), plus a bunch of intermediate levels. For example I can post 200 photos from my vacation in an album, and it just shows up as a single item in my friends' timelines. Any of my contacts can browse it if they want, but it doesn't take up disk space in their inboxes, they can start a conversation connected to any individual photo, and it's not as insistent as if I'd sent them in an email. And if I really want someone to see it, I can tag them or just message them with the link.
Facebook also serves as an auto-updated contact list. If I update my email or phone number, many of my contacts won't have the new information. FB doesn't have that problem. In fact I can put my phone number and email on my FB profile and my contacts can optionally auto-sync that info right to their phones so it is always up-to-date. It's possible to be "friends" with someone but not "follow" them so you don't see their posts in your normal timeline but you still get their contact info. You can also make closed or open groups which are helpful for event planning and multicast messages, and "friend lists" which let you customize the reach of each post. I guess you could do that last one with email too.
Being able to easily react to any status that your friends post, and being able to comment on that. The email equivalent would be a separate email thread for each status update from each friend. That would quickly get messy.
And a big one for me is being able to create an event, and invite friends to it. Any updates to said event can then be communicated to all those interested without you having to make updates to a list of email addresses.