As far as I understand:

1. You're on a website about pets, which has FB code on it: Facebook can't link that activity to your account anymore, which means they'll have less accurate info about you that might be relevant to advertisers.

2. You see an ad on Facebook and then go to that website to make a purchase -> FB can't link that purchase to your profile and as such a) can't tell the advertiser whether their ads are effective b) can't use that data to optimize the campaign for users like you who might also be likely to buy.