In Bloodborne, I had 12 Insight and 12 Madman's Knowledge.
At first the Insight Bath Messenger had for purchase:
The bells for co-op + Pungent Blood Cocktails; and Father Gascgoine's attire, which wasn't very good.
I just defeated a boss…
the Blood-Starved Beast, which unlocked some okay attire
Which I bought, along with a couple of items, to drop me back to 4 Insight.
At 15 Insight, the difficulty increases.
So if I use up all of my Madman's Knowledge now, I'd be at approx. 19 Insight. Would that then make any new stuff available for purchase at the Insight Bath Messenger; to deplete my Insight level back down below 15?
Because I don't see a reason to keep a high Insight level; therefore I'm just stocking up on Madman's Knowledge atm.
Update: After reaching 15 Insight, I bought all of the stuff I originally didn't want, purely to get me back below 15…
Update 2: After buying some additional crap I don't need, to get the Insight back below 15; my Insight eventually got to 16 again naturally. I've decided against buying any more crap from the 2nd Bath Messenger, and trying out the game at a harder level… maybe it has some benefit? It appears nobody knows…..
Best Answer
No. Insight does not influence stock at all, it only unlocks the shop when you reach 10 ( 1 in latest version) Insight and lose it goes under.
If you are looking to unlock stock you get all of it by:
Slightly off topic lore side:
Insight has tiny effect on game difficulty and is mostly there to keep the player immersed into game's lore. According to the game's lore insight is gained by learning about the world and coming into contact with the Great Ones (basically seeing what you shouldn't see) and allows to perceive more things. It's sort of interesting how some enemies appear and other enemies start using new attacks, that they might have been there, but none of it hurt you because you couldn't perceive them. Also, too much insight enables perception beyond what people can handle therefore driving them insane and there are references scattered throughout the game.
At any rate, if you are interested in lore I am sure someone has analyzed and written about it.