Almost always, a person seeking an annulment is someone who has been married, is now divorced for six months and wishes to marry again, specifically in the Catholic Church.
Divorced people, no matter what their religious affiliation, have a carefully protected right in this Church to ask the Church to determine whether or not their previous marriage was valid. If they are not of the Catholic faith, they seek this generally because they wish to remarry, and the intended spouse is a Catholic who wants the marriage to be recognized by the Catholic Church. We respect the vows of marriage of all people, no matter what their religious affiliation is. Members of the Catholic Church, however, are bound to have their marriage recognized by the Church. This is why members of other churches must often go through an annulment process before they can marry someone in the Catholic Church.