At the top of Elden Ring's Divine Towers lie the bodies of deceased Two Fingers beings. Despite having passed, these magical creatures are capable of reactivating the runes the Tarnished collects from Shardbearers, who are the demigods who stole pieces of Elden Ring after the Shattering. What brought the Two Fingers to these locations, what killed them, and what grants them their power beyond death is one of the game's biggest mysteries.
However, the primary purpose of the Two Fingers may hold a clue as to why the Two Fingers are located in the Divine Towers. According to the Two Finger Heirloom, the Two Fingers have a connection with the Greater Will that allows them to communicate the "words of faith" of the Golden Order to the Finger Readers who can understand them. In this way, the Two Fingers translate the desires, demands, and power of the Greater Will to the Lands Between.
This last note about power is particularly important about the Two Fingers, as it may hold the key as to why they were placed in Elden Ring's Divine Towers. The Greater Will was responsible for the creation of the Elden Ring, investing in it a power that is made up of individual runes (e.g., the Rune of Death that Marika removed). These runes are the shards the demigods took after the Shattering, and are also what the Two Fingers of the Divine Towers activate. What this could then mean is that the Two Fingers atop the Divine Towers may have been interconnected with one another (and the Elden Ring) to harbor the power of these individual runes in what were meant to be safe and sacred locations. The Shattering then broke this web of the Two Fingers and left them vulnerable.
How The Two Fingers Could Have Held The Power Of The Elden Ring
Supporting the possibility that the Two Fingers atop the Divine Towers held the power of the runes is that the only Divine Tower to be missing Two Fingers - the Divine Tower of Elden Ring's Liurnia - hosts Ranni's corpse. Marika removed the Rune of Death from the Elden Ring in coordination with the Greater Will, but Ranni stole it in the Night of the Black Knives to place half of the Hallowbrand - the mark of Destined Death - onto her flesh. That the Two Fingers are missing from this Divine Tower shows that their removal also caused the removal of the Rune of Death from the Elden Ring; that Ranni's body lies in their place shows that Divine Towers are where the power of the runes are harbored within a chosen vessel.
Additionally, the location of the Divine Towers in relation to Shardbearers indicates the Two Fingers once held the powers of the runes. The runes of the Shardbearers can only be activated at the Divine Tower closest to them: Godrick's Elden Ring Great Rune can only be activated at the Limgrave Divine Tower, Radahn's at the Caelid Divine Tower, etc. That the Two Fingers are slaughtered in these locations and are able to activate the rune the closest Shardbearer holds suggests they once held this rune within themselves. Whether they died as a direct result of the Shattering or whether the demigods killed them to obtain the runes, it appears they likely connect the runes to the Elden Ring through the Divine Towers and continue to do so after their deaths in Elden Ring.