NC drill files are used by Numerically Controlled drilling machines to drill the right number of hours with the right diameters in the right places. You do not need to export them from EasyEDA because when you press the `Fabrication Output` button in the PCB Editor that file along with all the others needed for PCB fabrication - whether directly by EasyEDA or by a 3rd party fab house of your choice - are automatically created and zipped up for you. You only need a drill for it you have an NC machine and you want to grab your own PCBs. However given the less cost and professional quality (plated though holes and multilayer board) that you get from EasyEDA, it is hard to understand why you might want to try to make your own PCBs.