The Catholic Church teaches that a valid marriage can NOT be undone, period, that's it. In order to understand the need for anulments in the Church, you have to understand that first and foremost. You don't have to agree with it, but you have to understand it. Marriage is a sacrament to Catholics, just like baptism, confession, holy orders, etc. Once a sacrament is done, it CAN NOT be undone, period, that's it. Nobody can be unbaptized, nobody can be unconfessed, and nobody can be unmarried (according to the teachings of the Church.)
Now, if the marriage or wedding wasn't done validly, then to declare that there was no sacrament is not undoing the marriage, it's proclaiming that the sacrament did not occur. (The Catholic Church is very specific about how things need to be done...if the proper form isn't followed, a sacrament can be invalid.) This is all that an annulment is, a declaration that the sacrament of marriage did not occur.
Is it abused and overused in the US? Absolutely, but the question is, is that because the tribunals making the decisions are too lax, or is it because so many people are running off and getting married without following proper procedures and requirements meant to stop quickie marriages? (Catholics are required to attend premarital counseling and classes, etc, they can't just decide to get married one day and do it the next.)
But the reason they are necessary is to declare whether or not the sacrament actually happened or not. If they declare that it did, there's nothing anyone can do or say to undo that marriage sacrament.
I hope that helps explain what annulments are for.