Why Have The Segan Kohan Gadol?

There is a memra of the Rabbe Chanina Segan Hakohanim that appears in a few places in Shas, one of them being Yoma (39a.)  רבי חנינא סגן הכהנים אומר למה סגן מימינו שאם אירע בו פסול בכהן גדול נכנס סגן ומשמש תחתיו.  According to this גירסא the Gemorah is saying Why did the Deputy remain at the High Priest’s right side throughout the day’s service? Because if some disqualification befalls the High Priest, the Deputy can step in and serve in his stead.  However, Rashi brings another גירסא that the Gemorah is asking what is the point of the appointment of a Segan: ואני שמעתי מרבינו יצחק בר יהודה למה סגן ממונה שאם יארע פסול אבל כל זמן שלא יארע בו פסול אין הסגן עובד שום עבודה בעולם.  According to this approach the Gemorah seems to be saying the whole purpose of the Segan is to be a backup. [It is a little difficult that Rashi says אין הסגן עובד שום עבודה בעולם for the Segan has jobs as Tosfos (15b) cites from the Yerushalmi.]

The Rambam Kli Mikdash (4:16) says מְמַנִּין כֹּהֵן אֶחָד יִהְיֶה לְכֹהֵן גָּדוֹל כְּמוֹ הַמִּשְׁנֶה לַמֶּלֶךְ וְהוּא הַנִּקְרָא סְגָן. וְהוּא הַנִּקְרָא מְמֻנֶּה וְיִהְיֶה עוֹמֵד לִימִין כֹּהֵן גָּדוֹל תָּמִיד וְזֶה כָּבוֹד הוּא לוֹ. וְכָל הַכֹּהֲנִים מִתַּחַת יַד הַסְּגָן.  It is clear that the Rambam understands the point of the Segan isn't to be a mere backup, its to be the Kohan Gadol's valet.  So, he must have learnt like the first גירסא.  The Rambam goes לשיטתו in the Laws of Avodat Yom Kippurim (1:3) that says וּמַתְקִינִין לוֹ כֹּהֵן גָּדוֹל אַחֵר שֶׁאִם יֶאֱרַע בְּזֶה פִּסּוּל יַעֲבֹד הָאַחֵר תַּחְתָּיו.  No mention of the Segan, any Kohan can be appointed to be the backup.  [This raises difficulties in the Gemorah Nazir 47b as the Briskor Rav there points out, ואכ"מ.]

The Briskor Rav tries to read even into the second גירסא that it also agrees the main job of the Segan is to serve the Kohan Gadol, the Gemorah is only asking why do they designate the Segan in place of the Kohan Gadol on Yom Kippur.  However, it doesn't seem that way from the Rishonim.  Tosfos Yeshanim asks why would they need a Segan in Bais Rishon, they had the משוח מלחמה?  He answers that there will be איבה for the משוח מלחמה already has some of the dinim of the Kohan Gadol.  But we see from the question that he assumes the whole point of the Segan is merely to be a backup, not to serve the Kohan Gadol.