Nnmartin did a thread on this - saying quite vehemently that marriage should be abolished. As I said in that thread, I'll say in this one again. Marriage absolutely has a place in society, and is important on so many levels. As an atheist, I personally don't feel the religious significance of it, but objectively speaking, I see how many attach their own religious views to the importance of marriage, and they must be recognized.
But in my view, more importantly is the legal foundations behind marriage. Being married is a question of your status within society, and it greatly affects the way you live in a society. There are differences in taxation, legal rights, ownership etc. This is why it is important that more societies legalize official unions between same-sex couples. More than anything, in my mind, marriage is a legal, recognized union that grants the people within it certain rights and privileges - and therefore it has a definite place in society.
Culturally too, outside of religion, marriage is an expression of deep committment between two people, and while it seems meaningless in the eyes of some, in the eyes of others is absolutely meaningful. If a number of people wish to express their love and ocmmitment to each other through marriage, then they should do so! And society must encourage it, and accept it. So, for this reason too, marriage has a place within society.
As I mentioned, I don't really hold with the relgiious reasoning behind it, but I'd be a complete hypocrite if I didn't tolerate and accept it as reasonable and justifiable. So all of the above, as far as I can see, give the concept of "marriage" its place within society.
<cue rapturous applause>