Why grant an annulment? There is scripture that in the case of adultery one can be granted a divorce and then one can remarry.
Matthew 5:31-32
I don’t have the verses memorized, but Jesus said divorce was not a sin if there is adultery or abandonment.
The Catholic Church will grant annulments because abandonment, adultery, severe abuse, extreme deception (spouse deceived other spouse into marrying under false pretenses), one spouse refusing to have any children.
After I legally divorced my first husband because of adultery, I eventually met my second husband who is Catholic. To be married in the Catholic Church I had to get my first marriage annulled. I filled out what must have been a fifteen page questionnaire and submitted it to the local diocese. Because of the high volume of annulment applications, I waited three years for a decision, finally got my annulment.