Celestials are a strange piece of D&D/Pathfinder. They are a necessary group, serving as the compass opposite to devils and demons. Sometimes they serve to provide context or boxed text to the characters. And lesser celestials can be summoned for a few rounds to fight in combat for your cleric. Aside from that, however, they seem to provide only one other value proposition to game play. And that’s when they fall.
So many modules and campaigns seem to have fallen angels as a key opponent or story driver. At times it almost seems that PCs face off against fallen angels as often as they do devils or demons. But they aren’t even always foes. Search the web for “D&D fallen angel” and you will find all kinds of templates and rules that people have created to turn fallen angels into PCs they can play in game.
The theme of the fallen angel seeking redemption for their crimes is a common one. I’d even say it’s become so common in gaming, literature and media as to achieve trope status. There’s just something that resonates with that theme in each of us. And I can see why. It’s a very human feeling to want to make up for past mistakes and the fallen angel falls right into that niche.
Playing one as a PC gives you a chance to play that out in game. But even when it’s an opponent who is the fallen angel, it’s often that the PC’s will want to find a way to redeem them rather than kill them. For example, when we played through the Shackled City adventure path, on Occipitus there is a fallen angel who is the guardian of the Test of the Smoking Eye. I remember, playing as Aeduin, that I wanted to try and find a way to find redemption for that guardian. As I recall, we did not succeed in that endeavor. But it was the first instinct to do so rather than kill them. Perhaps, if I’d played it differently, the result might be different.