How purity w/o the specific optimizations then allowed is useful ?

[Basile B.]

Short story for the context : someone has recently proposed the 
concept of purity for FreePascal, and only for their own dialect 
(aka _ObjFPC_). The proposition is to "check purity only when 
optimizations are enabled, because essentially the point of 
purity is only to optimize more" (think CTFE, think memoization).

But [I fail to explain the 
opposite](https://gitlab.com/freepascal.org/fpc/source/-/merge_requests/645#note_1863564617). How is purity a useful concept if not leading to specific optimizations ?

And is that even the case ?

Side note: I dont plan the use your answers to arguee w/ the 
author. It's more that conceptually I'm interested.