By default, Keeping reverses the order that email gets "stacked" in your Gmail inbox. This means that in a back and forth email conversation, the newest email will be "on top" of the stack, and you will reply from the top. We believe that being able to reply without scrolling to the bottom is a big productivity boost, so this option is enabled by default. You can change this option in your Preferences.
If you would prefer to use Gmail as it behaves normally (where new emails arrive "on the bottom"), you can change this in your Preferences.