Rev 5:8 appears to be an allusion to Ps 141:2 which says this:
May my prayer be set before You like incense, my uplifted hands like the evening offering.
Thus, the prayers of the righteous have always been represented by the metaphor of incense.
Thus, in Rev 5:8, all heaven is interested in the prayers of the saints; not that God needs to be reminded about what the saints are saying; as with all else in Revelation, it is a metaphor for showing the participation of all heaven's personnel in the process of God's salvation.
Ellicott says this:
It is not the Church alone which is interested in the revelation which will throw light on life’s mysteries and the delay of the kingdom: the whole creation groaneth, waiting for the reign of righteousness; and therefore the four living beings, who represent creation, join with the elders, who represent the Church, in the adoration of the Lamb who holds the secret of life’s meaning in His hand. The vials (which seem to be censers, as they hold the incense) and the harps, it is perhaps more natural to suppose, were in the hands of the four-and-twenty elders, and not of the living creatures. Here, then, we have the praises (represented by the harps), and the prayers (represented by the censers) of the world-wide and age-long Church of Christ.
Indeed, the 24 elders and the four living creatures then use the prayers of the saints (represented by the incense) as a further excuse to praise God and the Lamb as recorded in Rev 5:9, 10.