iOS 8 Uses Safari Passwords for App Login
Thanks to Safari saving your passwords, you can login to apps easier. As noticed by 9to5Mac, when you sign in to a site, save the password, and then download the app it will notice that and ask it you want to use those credentials. For example, if someone signs in to the Gmail site on their phone, saves the password, and then downloads the Gmail app, it will ask if you want to use that password in the app.
Currently, this is a feature that was just shown off at WWDC. However, Apple is allowing developers to use an API to implement this feature into their own apps.