The USB-C to USB Adaptor connects a Mac that has a Thunderbolt 3 (USB-C) or USB-C port to a device that uses a USB-A cable.
To connect an iPad Pro or iPad Air with USB-C to the USB-A port on a Mac, use a third-party cable, such as the mophie USB-A Cable with USB-C Connector.
Data transfer and charging other devices
USB-A 
Use the USB-A port of this adaptor with flash drives, cameras, USB hubs and other devices that connect to USB-A for data transfer or charging. It supports data transfer speeds up to 5 Gbps (USB 3.1 Gen 1).
For example, you can connect the Apple Lightning to USB Cable to this port to sync and charge your iPhone, iPad or iPod. Or use the Apple USB-C to Lightning cable to connect your iOS device without an adaptor.
This adaptor requires no power to operate, but devices that you plug into it might draw power from your Mac. To connect to USB-A devices and charge your MacBook at the same time, you can use the Apple USB-C Digital AV Multiport Adaptor or Apple USB-C VGA Multiport Adaptor, which also provides a USB-C port for charging.
