Maybe you've already played Liberty City Stories on the PSP. Maybe you think the lack of multiplayer modes mean the PS2 version isn't worth playing, because you've already seen everything the game has to offer.
You've got a point, but we're willing to bet you haven't seen everything. Not only are signs, posters and license plates much easier to read on a full-sized TV, but you might be surprised at the things you missed by not having a second analog stick to look around.
For example, you might not have noticed this the first time you played:
Keep looking around, and you're bound to see plenty random things that were all but invisible in the handheld version.