I believe the reason that God allowed some OT saints to rise from their graves at this time was because the restoration of the people of God was described in the OT prophets as a resurrection from the dead. The people of Israel in sin and exile are described as being dead.
Baruch 3:4–8 (RSVCE)
4 O Lord Almighty, God of Israel, hear now the prayer of the dead of Israel and of the sons of those who sinned before thee, who did not heed the voice of the Lord their God, so that calamities have clung to us. 5 Remember not the iniquities of our fathers, but in this crisis remember thy power and thy name. 6 For thou art the Lord our God, and thee, O Lord, will we praise. 7 For thou hast put the fear of thee in our hearts in order that we should call upon thy name; and we will praise thee in our exile, for we have put away from our hearts all the iniquity of our fathers who sinned before thee. 8 Behold, we are today in our exile where thou hast scattered us, to be reproached and cursed and punished for all the iniquities of our fathers who forsook the Lord our God.’”
Again in Ezekiel 37 Israel is described as a valley of dry bones.
When the prophets speak of the restoration of Israel they use the image of a resurrection. See for example Hosea 6:1-2)
Hosea 6:1–2 (RSVCE)
1 “Come, let us return to the LORD;
for he has torn, that he may heal us;
he has stricken, and he will bind us up. 2 After two days he will revive us;
on the third day he will raise us up,
that we may live before him.
Psalm 85:6 (RSVCE)
6 Wilt thou not revive us again,
that thy people may rejoice in thee?
There are other passages like that but I think the most important one is from Ezekiel
Ezekiel 37:11–14 (RSVCE)
11 Then he said to me, “Son of man, these bones are the whole house of Israel. Behold, they say, ‘Our bones are dried up, and our hope is lost; we are clean cut off.’ 12 Therefore prophesy, and say to them, Thus says the Lord GOD: Behold, I will open your graves, and raise you from your graves, O my people; and I will bring you home into the land of Israel. 13 And you shall know that I am the LORD, when I open your graves, and raise you from your graves, O my people. 14 And I will put my Spirit within you, and you shall live, and I will place you in your own land; then you shall know that I, the LORD, have spoken, and I have done it, says the LORD.”
Dr Scott Hahn writes:
‘‘Early prophets caught a glimpse of God’s plan to raise Israel up from the dead (Isa 26:19, Hos 6:2)…Resurrection can be used as a metaphor for ‘‘restoration from exile’’ (Ezek 37:11-14).’’ (Scott Hahn, Catholic Bible Dictionary, p.768)
‘‘Hosea depicts Israel’s restoration from exile as a third-day resurrection (Hos 6:2). Since the Messiah represents Israel in the fullest sense, embodying both its vocation and destiny, Christ’s own Resurrection initiates the resurrection of Israel from a state of spiritual death (Rom 11:15, 25-27).’’ (Scott Hahn, Icsb, p.155)
N.T Wright also remarked:
‘‘Ever since Ezekiel 37, the idea of resurrection had been used as picture language for the true return from exile’’ (Tom Wright, Luke for everyone, p.188).
So given all this I believe that by allowing these OT saints to rise God is showing that the cross has brought about forgiveness of sins and now the restoration of the people of God is beginning. The cross of Jesus is what accomplishes this. The Holy Spirit had revealed this to Ciaphas
John 11:49–52 (RSVCE)
49 But one of them, Caiaphas, who was high priest that year, said to them, “You know nothing at all; 50 you do not understand that it is expedient for you that one man should die for the people, and that the whole nation should not perish.” 51 He did not say this of his own accord, but being high priest that year he prophesied that Jesus should die for the nation, 52 and not for the nation only, but to gather into one the children of God who are scattered abroad.
And as Jesus himself says.
John 12:32 (RSVCE)
32 and I, when I am lifted up from the earth, will draw all men to myself.”
Finally then. Matthew 27:52 is a historical even that prefigures what God will do in restoring his people. Not only Israel but also all mankind through his universal Catholic Church. This is why pauls says that when we are baptized, we are raised from the dead (Col 2:11 ff, Rom 6:3-4)