For example, without going into an industrial perspective but limiting ourselves to a fairly simple setting such as Building Automation in an office complex, there may be hundreds of sensors to measure the temperature of environments, to control lighting systems, to monitor the presence of people, to define any areas with privileged access, to acquire data on environmental pollution, to manage security systems: therefore, not only sensing but also control functions. And all these devices can use different protocols.
Remember that a protocol, in a general sense, is a formal set of conventions that regulate the format and timing of a data exchange between two "terminals". The functionalities required to a protocol concern the formation of a connection and its destruction at the end of the communication, the management of connection commands, the management of buffers intended to collect messages for their subsequent sending to destination, the control of the correctness of transmitted messages, the functions of signaling anomalies in transmission or reception, the conversion of code for sending the message on a given transmission medium and its subsequent "reconversion" to make it understandable to the destination.
If a protocol is a set of rules, a network protocol is the set of rules followed by a network: Network Protocols are standards made of rules, procedures and data formats that define the communications between the devices of a network.
In an IoT context, it is a fact that there are heterogeneous network protocols such as WiFi, Bluetooth, Ethernet, MQTT, ZigBee, and others; moreover, each one could refer to different control environments and have different management and security models.
Also, many sensor elements are "low energy" and could not support "energy intensive" protocols such as WiFi or Bluetooth. Finally, some devices generate by their nature several packets of data that their aggregation becomes if not unmanageable at least challenging, and not all these data are useful in their raw form.