It is easy to forget how much weaker the support was for gay marriage was in 1996 than it is now. State after state was passing constitutional amendments banning it. I don't think I would claim it was created to spread tolerance but I do think it was created to lessen the fear many people had. People needed time to adjust to a new ways of thinking about something as basic as sexual identity.
If that goes too far then I probably should skip my theory that despite, or more accurately because of, how horrific AIDS is it forced people to stop dehumanizing gay men as strangely dressed effeminate and somehow threatening perverts and to look at them as people who loved and lost the same as everyone else. Of course not everyone got it and the true haters saw it proof that all their hate was valid but more and more people stopped dehumanizing gays and then more gays started coming out of the closet and turned out to more similar than different people started to fear the change less. I wouldn't be surprised to have to explain to grand children why we ever needed a phrase like "coming out of the closet" and for them to look on it the same way I look segregation and wonder how people ever thought that was OK.
Without the room DOMA gave them I think more and harsher laws would have been enacted and if people thought they would be forced to accept gay marriage I think there would have at least been a real attempt to pass an amendment. I think it would have failed but it would have made everything harder. There points in history where these things have to be pushed but there are also points where you need to let the affects of the previous pushes develop.
I do think same sex marriage will be legal in all 50 states in the not to distant future but I do think that without DOMA it would likely be a more distant future. I also think there is still a huge amount of fear and misunderstanding to get past but I think we have reached a point where the haters will start to be ostracized more than the people they hate. In fact I think we are passed that point.