What USB 3.0 functionality does Intel's iusb3mon monitor?

To directly answer your question,

The job of this utility was to monitor the functionality and system usage of your USB ports in the background and look for any possible problems. All Windows Operating Systems have a core USB Monitoring system already and the Intel Utility is not necessary in any way. Intel wanted to use a tool Intel Made to monitor USB related system problems etc. But it isn't necessary and can be stopped and removed.

Intel does not publish any information on iusb3mon, except to call it a "monitor". One has to go to the USB 3.0 implementation of Renesas Electronics in order to find the explanation.

In the release notes of D720201 & D720202 Design Resources Release it is described as :

The Monitor Application, if installed, alerts the user when a USB 3.0 device is connected to a USB 2.0 port. Performance normally will improve if the device is connected to a USB 3.0 port.

This is a monitor, not a driver. It is probably activated by the connect-device event, so should not take much resources. If it starts hogging resources, it can be uninstalled without any problem. Both USB 3.0 and 2.0 devices will continue on working correctly with or without this service, as this alert is not essential to their correct functioning.