If for some reason your import fails and you need to reimport members again and you check the box that is for "Empty Local Data" the process should be smart enough to keep the VERY FIRST USER that you created when you installed IPB. Because if for some reason it fails on subsequent attempts to import, it may have imported records - none of which are YOUR admin account, nor the default account, preventing you from logging.

It keeps the user you're currently logged in as so that the import will finish, but beyond that... you're telling it to delete all members, that's what it will do, and that's what it's supposed to do.