This is a namespace used by the IDE tools. This is not needed to run the application on a device (or emulator). On the other hand, if you leave them, Android will ignore them when the application is running on a device.
This is used by e.g. lint and graphical layout tab.
You can check how it's used by lint here: http://developer.android.com/tools/debugging/improving-w-lint.html in section Configuring lint checking in XML.