As a basic consideration, the HMI is usually a Modbus master/client, for the sole reason that Modbus is a master/slave protocol and slaves are silent unless spoken to. If the state of a selector switch is changed on the HMI, it's up to the HMI, as a Modbus master, to write that change to the PLC.
If the PLC were the master it would have to continuously poll/read dozens/hundreds of status words looking for a state change on the HMI.