Some people here seem to be restricting the benefits of doing hijab to the individual, saying that it does nothing to change their sins 'in private.' What about the societal effects of having an entire population adhere to Islamic dress codes in public? After all, whenever the Qur'an discusses hijab, it does so in relation to how other people view those who do hijab, not an individual's own sense of piety or holiness.
"And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what (must ordinarily) appear thereof; that they should draw their khimār over their breasts and not display their beauty except to their husband, their fathers, their husband's fathers, their sons ....and that they should not strike their feet in order to draw attention to their hidden ornaments."
"Those who harass believing men and believing women undeservedly, bear (on themselves) a calumny and a grievous sin. O Prophet! Enjoin your wives, your daughters, and the wives of true believers that they should cast their outer garments over their persons (when abroad): That is most convenient, that they may be distinguished and not be harassed."
The reason given for the commandment to do hijab is societal in its very nature, and has less to do with an individual's 'Islamicness' or quantity of sins. So to say that hijab shouldn't be enforced because "you can't force piety on someone" is to ignore the reasoning behind the commandment, and the fact that the hijab ruling is inherently one of social relations with other people.