Aligning with Apple’s upcoming media event on October 22, Apple is now asking developers via e-mail to begin submitting their OS X Mavericks-ready applications to the Mac App Store for the review process.
Originally Posted by Apple
The reason for Apple's e-mail is to try and motivate developers to get their applications ready as soon as possible to make the upgrade for users as seamless as possible without issues regarding application compatibility.
OS X Mavericks will bring users a number of new features that app developers will be able to take advantage of. Some of the notable features in OS X Mavericks include App Nap, Finder Tabs, iBooks, and Maps among others.