> What are you protecting by making a throwaway?

Themselves. People will attack others on the internet for having an unpopular opinion that differs from their own. If your username is attached to your real name, then sometimes it's safer to post risky comments on an alternate account, instead of your real account. This is similar to when you have a public account and private account.

Also you can't edit/delete comments after a couple hours, and you can't delete them after someone replies to you.