I'd be willing to accept that it didn't have a significantly greater narrative point beyond demonstrating Saw's extremism - he was so far gone that he was okay with potentially destroying people's brains to satisfy his paranoia. It's why I mentioned witch-hunting - the whole thing is like that test where they would submerge the accused in a lake to see if she uses magic to save herself and just accepted any actual drownings as collateral damage in the war on witchcraft. In any case, I don't think it was guaranteed to destroy a person's brain, just that this was the worst-case scenario - the fact that Bodhi recovers sometime during the bit on the rainy planet suggests that he got off lucky compared to some of the monster's other victims.
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Iro's Top 100 Movies v3.0
I really just want you all angry and confused the whole time.