Suppose you're in a Hyper-V guest and are trying to use PCI passthrough. The ID_PATH that udev will construct for that looks something like "acpi-VMBUS:00-pci-b8c8:00:00.0", and obviously looking for "pci-" in the first four characters of that is going to not work.
Instead, strstr. I suppose it's possible you could have multiple PCI buses in the path, in which case you'd want strrstr, if that were a thing.