What kinds of simple expressions need a snapshot?

Hi,
I'm reading code in plpgsql, and I notice that exec_eval_simple_expr() will get a snapshot for the expr if it is in a VOLATILE function.
Then I try to find out why a snapshot is needed for a expr which has passed exec_simple_check_plan(). The only reason I could figure out is that maybe VOLATILE functions are involved in this expr, are there some other situation I missed?

Thanks.