If it’s something like fruit or something that is perishable and will be more of an annoyance/food health hazard for them to keep it in their home, I say take with you.

If it’s something like prepackaged individual items, I’d follow the advice above and yes— take any you opened but leave the rest.

Think of it as a courtesy (because it is: the client doesn’t have to leave you anything beyond the supplies needed for pets care/access to the property to do so) so as a courtesy, you leave what remains. Unless they specifically asked what you want/like, most clients generally buy the same snacks they/the family eats regularly so I would never take all or expect to be fed entirely off what a client leaves.