Instance Mail is a disposable (or throw-away) email service that lets you create an email address which you can use one time, and get rid of it when you no longer need it. You don't have to go to a separate website to check your mail, though. We forward it right to your regular inbox!
The main reason to use Instance Mail is to protect yourself against spam. Let's say you want to register on a forum or blog, but you're afraid they'll spam you (or sell your email address to spammers). Get a disposable email address from Instance Mail and give that to them instead! They can send you a verification email so that you can complete registration. Once you've registered you can turn off that Instance Mail address and never hear from them again.
Instance Mail can also be used to protect your identity. If your real email address contains your name or some other identifying information, you may not want to give it out to someone you don't know very well. Give them an Instance Mail address instead, and they'll never know where the email is being forwarded to! (Of course they'll find out your real email address if you reply to them, so for full anonymity you might just want to sign up for another regular email address).
There are many other good reasons to use a disposable email address, and there are a lot of disposable email providers out there. Unlike a lot of them, Instance Mail is backed by KisoLabs' robust and reliable email infrastructure. Just because you want to avoid junk email doesn't mean that you should settle for a junky email service. And of course it's free for as many email addresses as you want to create!