If you'd like to modify the location of your event, editing it erases all the information previously input, leaving only the optional name field. The form should pull up this information for easier editing.
@Samat, this should be fixed now. I've just tested from an admin and a test account. I created an event with a location, saved it, hit the Edit button below the post, and selected the location for editing. All previously entered fields appeared (rather than just the "Name optional" one). Give it a try and confirm! (For reference, here is your original report: https://discuss-space.wmflabs.org/t/edit-the-location-of-an-event/958/3).