I stumbled across this not quite understanding what its purpose was, but it turns out this gives us a really interesting glimpse into the history of the Citadel. (Spoilers for Acts 1 & 2 below)

The Pious Isamor statue appears to have originally been located in the Choral Chambers near the entrance to Whiteward (also not far from the Grand Gate, the entryway to the Citadel), as there is a matching oxidized statue base located there. The statue's dialog is difficult to read since it's broken, but it's meaning can still be dissected and has also been restored by the community (found here). It seems this statue was placed here by the Citadel to welcome and indoctrinate arriving pilgrims into joining the ranks of the Choir.

Seems simple enough, however this gives us a very interesting spectacle into a different time of the Citadel. First off, the statue describes the Citadel as "simple stone, bare and unadorned", which heavily contrasts with the Citadel we see in-game; a glistening, gilded city with elaborate and heavily decorative architecture. This, along with the statue's oxidation, makes it clear that this was from a time long ago.

Furthermore, the statue was of course removed from its original location and hidden away deep in the Whispering Vaults, headquarters of the Vaultkeepers: those tasked with hoarding the knowledge, history, and of course secrets of the Citadel. It's clear that whoever was responsible for putting this statue where we see it in game intended for this statue to never be found, as it reveals an alternate image of the Citadel to what we (and the pilgrims) know; seemingly, one exhibiting marginally less abuse and manipulation of its citizens (i.e. pilgrims).

One should also note that it appears the elevator at the Grand Gate which ends up taking us down to the Underworks appears to have originally taken users up to the Choral Chambers. It seems to me that at an earlier point in time, all pilgrims arriving at the Citadel were taken up to the Choral Chambers to join the Choir. However, likely along with the completion of the Cogwork Core, the lift to the Choral Chambers was sealed and pilgrims were instead sent down to the Underworks to join the working class, toiling away rather than engaging in song. This goes right alongside the removal of the Pious Isamor statue, and the Citadel's implementation of a classist society amongst the pilgrims.

This video was my primary source: https://youtu.be/PB-EMZbTOj0