Because I figured 'why not'.

We had been together for a year & were talking about having a baby (I already had Panda), my younger sister asked if she could be a flowergirl when we got married & that kicked off the conversation.
Not to sound too fickle about it but I also knew of couples who had been together 7+ years, gotten married & then separated a year or so later. I thought there wasn't much point in waiting if you were comitted, you may as well go ahead & do it.
I was 19 and I knew that we would change & grow over the years.. I said to my uncle on my wedding day if we got 5 good years I figured it would be worth it. We had our 14th wedding anniversary this month, so I guess it was worth doing

ETA - as for changing things - it didn't really, although I think possibly it did make me feel more cemented within the relationship. Maybe. Our relationship has changed an awful lot over the years though, it is definitley not the same as it was when we married.