Bestiary 2 contains updated monster rules, including several new abilities that aren't explained in Bestiary 1. Quickie example: Lifesense, though admittedly there's not much to that one. But Lifesense is an ability listed in the Universal Monster Rules section of 2, and it's not in 1.
2 also has a few other updates such as the extended table for build-your-own-monster rules to CR 25.
Pretty minor updates, mostly, and of course the most recent rules can be found online in any case- but that is a reason to have Bestiary 2 in addition to 1 at your game table (especially if you can't easily get online from there).