Whats the reason that the Praxic Order uses Pyramid structures as temples?
Mainly to build over them and cleanse them of Darkness. Praxics were always about just destroying the Darkness and not really into understanding it, so claiming Darkness structures and converting them can serve as a way to take over, remove the Darkness and keep it at bay by turning these structures into temples dedicated to the Light. These structures were also places where they could keep relics of Darkness away from the City and in a place where they could easily be contained and not a danger to anyone. From the start of the Fire and Ice exotic mission:
Aunor: The Praxic Order maintains vaults throughout the system, to purify or lock away dangerous artifacts of Darkness. Only Praxics who’ve earned a Cormorant Seal are allowed entry.
There is a bit of hypocrisy in this that's mentioned in-game, by Eris, at the end of the exotic mission:
Aunor: Superb bladework, Guardian. You’ve taken your first steps towards Praxic dogma… For whatever the hell that’s worth. It seems the Order… that we created Bael in the first place. Eris: You methods armed Bael. Help us finish him. Aunor: Of course I will! I just didn’t think— Eris: That your zealous Order could make a mistake? That hoarding what you claim to denounce would cost humanity? Aunor: We were doing what we thought we had to. To protect the City. Eris: So were all of us, utilizing the Darkness against the enemies of humanity — while you excoriated us for it. Aunor: I… need to meditate on this. Guardian, there is no Dredgen your blade can’t cut. It’s a reflection of you. Today, I’m not proud of myself or my Order. Perhaps you can help us change.
This is one of the moments when Aunor begins to realise that she may have to change her mindset and her Order, something that continues to happen as we go through the temple in the Equilibrium dungeon, especially at the end when she wonders if the Praxic relics will accepts a mix of Light and Darkness and it does.
