Purpose 

Changes in costume can be done for purposes of changing the character direction in stories, or simply as a means of publicity.   This generally refers to substantial changes in a character's costume, not just tweaking in terms of specifics based on artist preference.  For instance, the change in the golden age by Wonder Woman from wearing boots to sandals would not be considered a costume change.  
 

The New 52

 In mid-2011 it was announced that an editorial decision had been reached to reboot the entire DC Comics lineup following the events of Flashpoint.  In so doing, all of the series were reset to issue #1 and every character's design was modified.  This caused some concern among some fans that certain female characters were being drawn in a more revealing manner. 
 

Marvel NOW 

After DC rebooted its line Marvel decided to follow the same model in the following year and several prominent characters were also redesigned.