Yeah.....it probably ejects discs from the optional USB optical drive.
how else would you open the try(if its not slot load), you think apple is going to release a drive with..*GASP* a BUTTON?!!?
this can by like asking why there is an eject button on the macbooks now that have slot load drives, theres no need for an eject button, just drag the icon to the trash or click on the little eject arrow
some people (including me) still like having buttons to push