Punishment for sin
The opportunity to gain indulgences is a gift from Christ through his Church, and should not be dismissed as unimportant. We need all the help we can get! We should constantly strive to be so pure in God's sight that were we to die we would go straight to heaven. Indulgences aid in this, and can make all the difference between the soul being burdened with a debt of punishment for sin, or being free from this.
Above all things we should give glory to God, and glory is given him when we repair the damage done by our sins. Indulgences help in this.
Not only that. Charity for the souls in purgatory is shown by striving to gain indulgences for them. We can help them in this way, and shouldn't ignore the opportunity. They are in purgatory because they have not made full atonement for their sins, and they suffer greatly as they yearn for the vision of God. We can make some of that atonement through the indulgences we offer for them - full atonement if we gain a plenary indulgence. We should not neglect them in their need.
A helpful practice is to resolve each morning to gain whatever indulgences we can that day, whether for ourselves or for souls in purgatory.
A booklet explaining indulgences and giving the prayers and works required is available from Freedom Publishing and the John XXIII Co-op, price $4: New Regulations on Indulgences, by Winfrid Herbst, SDS. The John XXIII Co-op also has The Handbook of Indulgences, hardcover, 128pp, issued by the Vatican, which contains the new regulations, the indulgenced prayers and exercises, and Pope Paul VI's apostolic constitution Indulgentiarum doctrina, price: $22.)