As has been said, the mass and the wedding are two different sacraments, and some people just like to get both together.
A big difference is that the mass is celebrated by a priest, but the wedding by the couple. Originally, they could just do it alone (see the plot of Webster's Duchess of Malfi) but that led to so much confusion that a figure from the church is required by way of witness, but not necessarily a priest. There was a case last year where a couple were married (no mass) by a friend who was a nun — the bishop didn't object.
I can understand that. I think most religions went through the same sort of evolution. In judaism, you could once marry a woman in any one of three ways: intercourse, a gift, or a contract. But today all three are done together to make sure there is no misunderstanding.
I think that in the olden days, pretty much across the board, a man and woman simply decided to get married, and moved in together, and it was accepted by the community as a marriage. The problem IS that (even today) there's an awful lot of scoundrel men who will tell a woman anything to get her into bed with them, including I love you and lets get married. Then they just disappear into the night. Or they'd say she is lying and trying to trap them. Especially if she gets preggers. That's when having witnesses to vows started becoming the norm.
It's also of course not unheard of for a woman to "hear what she wants to hear." Many women assume that if a man sleeps with them it means they are loved and have a committed thing going on, when nothing of the sort has ever been said. For these women too, the sheer obviousness of a wedding ceremony makes things clear.
