Shadow of the colossus: Because of the increase in performance your grip drains a lot lot faster in this version (apparently due to the constant frame rates) so the game, specifically hard time trails are almost impossible to beat in the current state.
Ico: There is a trophy for beating the game in under 2 hours, the current best time is a little under 20 minutes under that but that time used some time skips, another nearly impossible trophy.
They're good for re-playability. You can use trophies as markers for improving your achievements. That's how I use them. Others might use them for a "mine is bigger than yours" peeing contest, and post their achievements on forums etc, but you don't have to use them like that.
Seriously... don't torture yourself, mate. A few people have related the theory to me that several trophies in the Ico collection are actually impossible, owing to the increased frame rate or something. One had entered into a dialogue with the people at Sony, but I don't know what came of it. I don't know where there is a list of high achievers, but I'm sure I read that someone saw it and no one had the platinum. Which would support the impossible trophy theory.
as i recall in shadow of the colossus everytime you complete the game you can reload completed save data and start anew with the exception of retaining your increased grip.the grip gauge increases after defeating a colossus in fact thats the only way you can to get to the secret garden by completing the game multiple times