Thanks for the question.
Annulments are made as an assessment of the sacramental nature of the union.
- In other words, was it a valid marriage?
To determine this, an investigation is made to consider any impediments to the free exercise of the will at the time of the union. So, for example, if your husband were to have been hiding issues from you related to sexual impurity, infidelity, and impotency, they could be grounds for an annulment, because your free will was compromised by a deception. It is sort of like fraud. But you can't sort this out by wishful thinking either. A tribunal is set up within each diocese to look at all the circumstances around the time of the wedding and determine if there were legitimate impediments that foiled the sacrament. Pressure, deception, canonical irregularities, form, etc., are all taken into consideration.
If the marriage was legitimate, and your husband later decided to be unfaithful, that doesn't negate the marriage—just his character. Basically, an annulment says, there was no sacramental marriage to begin with.
Since Christ taught that divorce is not an option for Christians, for sacraments are permanent bonds, you definitely would want to have this sorted out if you ever wanted to enter into another relationship and marry again. You can't remarry unless your husband dies, or it is shown that your marriage was nullified by an inherent flaw from the outset, and thereby you technically were not bound in the eyes of God.