Why is incense used during Mass?

The purpose of incensing and the symbolic value of the smoke is that of purification and sanctification. The usage of incense adds a sense of solemnity and mystery to the Mass. The visual imagery of the smoke and the smell remind us of the transcendence of the Mass which links heaven with earth, and allow us to enter into the presence of God.

Actually, the use of incense is an expression of prayer, and it is in fact very scriptural, very Roman Catholic, and very Judeo-Christian. There is a recipe for incense in Exodus (30: 34-36), and incense is associated with divinity and reserved for God (Ex: 30: 37-38). When we use incense and why:
According to the General Instruction of the Roman Missal incense may be used during the entrance procession; at the beginning of Mass, to incense the altar; at the procession and proclamation of the Gospel; at the offertory, to incense the offerings, altar, priest and people; and at the elevation of the Sacred Host and chalice of Precious Blood after the consecration.

Incense the Altar:
The Altar represents Christ and his five wounds indicated by the five crosses on the altar top. When we incense the Altar it also reconnects us to the original dedication of our church and to the angels and saints in heaven. If you attended the dedication of our church 24 August 2008, perhaps you remember after the Bishop covered the altar with the Sacred Chrism, he placed five braziers on the five wounds (crosses) on the Altar top. He poured much incense into the five braziers. This is a visual connection to the use of incense in the Book of Revelation. Just as the angels offer incense as a sacrifice at the altar of God, so to do we offer our prayers and sacrifices upon God's new altar. Not only is this the final step in the consecration of an altar, it also connects us, in the most perfect sense, to those gathered around the great altar in heaven. Furthermore, when we incense the altar at the beginning of the Holy Sacrifice of the Mass, we directly connect with Christ's Holy Sacrifice, for this is Christ's Altar of sacrifice containing his five wounds upon which his precious body and blood will be offered.

Proclamation of the Gospel:
What are we expressing with this incensing of the Gospel Book? It is a sign of devotion toward these holy words of and about our Savior, which have been preserved for nearly 2000 years, in reverence and love. Catholics firmly believe Christ is present in His word proclaimed, and so we bless and honor that Word who will imminently be present through His words, the Holy Gospel. As the sweet smoke (always first blessed by the priest) rises toward and surrounds the holy book, we are acknowledging: these are the words that God spoke when He became man and walked on this earth; this is the story of our salvation; here is the promise of everlasting life; here is the testimony of God's love for us.