Fortmatname field displays the name based on how you've configured Name Format within CB Configuration on the General tab. This is much more convenient as you can have various display types allowing for more unique display. Using JUST username may not fit sites who use name instead, using formatname field helps avoid such problems by allowing you to select the name display.

You do not have to display this field to users. Simply place it on a tab that is on a "not shown on profile" tab position. This way the field will still work, but no one will see it.