131 It is on this doctrinal basis that the cult of adoring the Eucharist was founded and gradually developed as something distinct from the sacrifice of the Mass. The reservation of the sacred species for the sick and those in danger of death introduced the praiseworthy custom of adoring the blessed Sacrament which is reserved in our churches. This practice of adoration, in fact, is based on strong and solid reasons. For the Eucharist is at once a sacrifice and a sacrament; but it differs from the other sacraments in this that it not only produces grace, but contains in a permanent manner the Author of grace Himself. When, therefore, the Church bids us adore Christ hidden behind the eucharistic veils and pray to Him for spiritual and temporal favors, of which we ever stand in need, she manifests living faith in her divine Spouse who is present beneath these veils, she professes her gratitude to Him and she enjoys the intimacy of His friendship.
Source: Mediator Dei (Vatican.va)