Why was the Angel seated on the stone at the Resurrection of Christ?

Christ is risen! Truly He is risen!

The Myrrhbearers beholding the Angels seated at the Tomb of Christ after His Resurrection (source)

 
Every word of the Holy Scriptures is placed appropriately with deep meaning, and we can perceive this if we struggle to obtain the humility and the illumination of the Fathers to understand how this is so. In the following short excerpt from St. Romanos the Melodist's Kontakion on the Resurrection of Christ (the beginning of which, "Though you descended into the grave, O Immortal One..." is still included in the Paschal service), he explains the significance of why the Angel was seated on the stone after Christ's Resurrection.
 

            Λαβούσαι θάρσος άμεμπτον εκ της φωνής του αγγέλου

φρονίμως αι γυναίκες απεκρίθησαν προς τούτον∙

«Αληθώς ανέστη ο κύριος, καθώς έφης∙

έδειξας ήμιν και τω ρήματι και τω σχήματι

ότιπερ ανέστη ο ελεήμων∙

ει μη γαρ ηγέρθη και επορεύθη της ταφής,

ουκ αν αυτός εκάθισας∙

πότε γαρ στρατηγός του βασιλέως

παρόντος κάθηται και διαλέγεται;

ει δε και τελείται εν γη τα τοιαύτα,

αλλ’ εν τω ύψει ουκ έστι ταύτα,

όπου θρόνος αθέατος και άφραστος ο καθήμενος,

ο τοις πεσούσι παρέχων ανάστασιν.»

   

The Holy Myrrhbearers coming the the Tomb of Christ, beholding the Angel, and the soldiers who became like dead men (source)

   

Taking unassailable courage from the voice of the angel,

The prudent women replied to him:

"Truly, the Lord has risen, as you said.

You indicate this to us by word and sign

That the Merciful One has risen,

For if He had not risen and proceeded from the tomb,

You would not be seated,

For when does the soldier before the King

Ever remain seated and speaking?

Though, therefore, things such as this are accomplished on earth,

In the highest, things are not like this,

For there is a throne invisible, and unspeakable, on which sits

He Who grants resurrection to the fallen."

   

(amateur translation, see here for the full Greek text)

   

The Angel proclaiming to the Myrrhbearing Women: "See the place where the Lord was laid..." (source)

   

Christ has risen from the dead, by death he has trampled on death, and to those in the graves given life!

Truly the Lord is risen!