The poll function is used to simulate the interrupts when the interrupt subsystem is not functioning (for example, when the system has crashed and is creating the system dump). The CAM subsystem sets the proper interrupt level before calling the poll routine. So all it needs to do is to call the interrupt routine (or the other way around, the poll routine may be doing the real action and the interrupt routine would just call the poll routine). Why bother about a separate function then? This has to do with different calling conventions. The xxx_poll routine gets the struct cam_sim pointer as its argument while the PCI interrupt routine by common convention gets pointer to the struct xxx_softc and the ISA interrupt routine gets just the device unit number. So the poll routine would normally look as:

static void
xxx_poll(struct cam_sim *sim)
{
    xxx_intr((struct xxx_softc *)cam_sim_softc(sim)); /* for PCI device */
}

or

static void
xxx_poll(struct cam_sim *sim)
{
    xxx_intr(cam_sim_unit(sim)); /* for ISA device */
}