Purpose of Gamma and Delta contexts

[mstarodub]

Hello, thanks for sharing this very readable implementation.
There's one thing that does not make sense for me at the moment: why do you use two separate contexts? I've seen this this in a couple of Lambda calculus implementations, but only found one reference to the technique.
I see that in your implementation, you use Gamma as a typechecking and Delta as a computational context, but could these not be merged into one?
Could you please explain the reasoning behind this segregation?