SCOTUS could once and for all rule on California's Proposition 8, a 2008 voter-approved ban on same sex marriage in The Golden State. The court could also take on the eight other cases that challenge the 1996 Defense of Marriage Act — a law that defines marriage between a man and a woman, thus denying federal benefits to same sex couples from states where gay marriage is legal.
It is possible, however, that the Court decides to leave the matter to the states — which would continue to approve or reject marriage equality through ballot initiatives or other measures. This would be a mistake since federal law supersedes state law and those same sex marriages from states where these kind of unions are legal would be invalid in states where there's no marriage equality (as well as at the federal level).
Most importantly, leaving the matter to the states would continue to deny federal benefits to same sex couples — effectively perpetuating government sponsored discrimination, as well as classifying LGBT families as second-class citizens. That is why SCOTUS should approve gay marriage once and for all.