This psalm is read on Good Friday because it contains phrases that are remembered in the gospel accounts of the trial and crucifixion. See verses 7-8 and 14-18.
The mood shifts in verse 22 from lament to assurance, "I will tell of your name to my brothers and sisters; in the midst of the congregation I will praise you."
The portion of the psalm that we are reading this week is the fulfillment of that vow--both parts of it, the praise and that the praise is not silent but before the congregation.
Philip didn't keep his gratefulness to God just between himself and God. I'm assuming that neither did the Ethiopian keep his rejoicing private.
The people that hear their praise directly will benefit from knowing about God. But not only the first hearers. They will tell what they have heard. And those hearers will tell. And so today, we continue to praise God and to praise aloud and widely.
Lectio Divina: Psalm 22:30-31 Posterity will serve hin; future generations will be told about the Lord, and proclaim his deliverance to a people yet unborn, saying that he has done it.