The doctrine of the session of Christ (sessio Christi) is required by the perfection of his one sacrifice. Christ "s[its] down at the right hand of God" because "by a single offering he has perfected for all time those who are sanctified."

Obverse of an early 14th-century
Bulgarian coin

Heb 10:11-14:"And every priest stands day after day at his service, offering again and again the same sacrifices [(ἕστηκεν καθ’ ἡμέραν λειτουργῶν καὶ τὰς αὐτὰς πολλάκις προσφέρων θυσίας)] that can never take away sins.  But when Christ had offered for all time a single sacrifice for sins, 'he sat down [(μίαν ὑπὲρ ἁμαρτιῶν προσενέγκας θυσίαν εἰς τὸ διηνεκὲς ἐκάθισεν)] at the right hand of God,' and since then has been waiting 'until his enemies would be made a footstool for his feet.'  For by a single offering he has perfected for all time [(μιᾷ γὰρ προσφορᾷ τετελείωκεν εἰς τὸ διηνεκὲς)] those who are sanctified."

     Contrast this with the doctrine of the session in Protestant scholasticism as summarized by, for example, Richard A. Muller, who, in the second, 2017 edition of his Dictionary of Latin and Greek theological terms, speaks only of the oppositional significance it takes on in the Lutheran and Reformed traditions, according to which it signifies, respectively, the ubiquitas (ubiquity) of the glorified body on the one hand, and its ubietas (whereness, locality) on the other.
     Undoubtedly Muller's sources didn't neglect Heb 10:11-14, despite the fact that the controversies of the late 16th century and beyond forced them to concentrate more on the significance of the (in Reformed terms) "location" (ascension to "the right hand of God") than that of the session itself.  See, for example, Heinrich Schmid, Doctrinal theology of the evangelical Lutheran church, pp. 380-407, which I have only skimmed, but where the emphasis seems to be on "the assumption, on the part of the human nature of Christ, of the full divine glory and dominion" (380), or, in Reformed terms, "1. Supreme majesty and glory" and "2. Supreme power" (Riis, 406), not (as here in Hebrews) the significance of the distinction between standing on the one hand, and sitting on the other.  But check also Heinrich Heppe (Reformed dogmatics:  set out and illustration from the sources) and, even better yet, Muller himself (Post-Reformation Reformed dogmatics:  the rise and development of reformed orthodoxy, ca. 1520 to ca. 1725, 2nd ed. (2003)), as well, of course, as the period before the Reformation!
     An enormous research program, no doubt.  For now, I simply set down the observation, and wonder (without starting down what is bound to be a very long path with many twist and turns) who (besides the exegete) treats the session itself in this way.
     The sessio Christi elsewhere in Hebrews:  1:3, 1:13, (2:8, 4:4, 4:16, 7:24), 8:1, (9:25-26,) and 12:2, at least.