Take a look at this report commissioned by the Canadian Council of Muslim Women:

http://ccmw.com/women-in-niqab-speak-a-study-of-the-niqab-in-canada/

http://ccmw.com/wp-content/uploads/2013/10/WEB_EN_WiNiqab_FINAL.pdf

The text of the longer PDF report concludes that the same reasons hold for hijab and niqab , i.e., as an identity marker focusing on difference.