Blue-and-orange morality is a thing. A given fey might well be Good, along that particular axis, but the risk associated with ticking them off might still be great enough that it presents a challenge to a Good party. Likewise the Ordning brings Good and Evil giants into the same structure, where a Good storm giant might have obligations to an Evil fire giant that bring it into conflict with the party.
Also, there's the notion that a stat block alignment might not be representative of every single individual. If you want your party to fight an Evil couatl, well, it's already statted out for you right there! And conversely, if you're taken by the stat block for the Solar and want your party to fight one, there's now a built-in narrative tension as to why this Celestial is acting this way.
(I know that some of the strictures of alignment have fallen away in modern D&D, but I'm comfortable ascribing these points at least back to 2014, probably further, especially with a creatively exploratory DM.)