Some people use plurk as their primary social media and check that before they check email. If I want to send a quick message, I know they will be looking at plurk before they look at email.

Sometimes it's a good way of reaching out to a few friends about an issue that you wouldn't want to make public - maybe a health issue, or getting advice from friends about a work-related problem

I might use them to reach out to someone about a project - hi, I'm doing this, are you interested in coming aboard. It's more relaxed than sending an email.

Just like Twitter DMs apart you can hit a couple of friends instead of just one

Oh ok cool thanks!