The much older model USB Camera Adapter is very limited in its capabilities. iPad alone cannot provide sufficient power for most USB accessories - such as USB Thumb Drives and Card Readers - and is limited to USB 2.0 data speeds. When connected to a USB device, such as a battery operated camera, the Camera Adapter only has to cope with transferring data.
If connecting a USB device that does not have its own source of power, such as a Card Reader and SD Card, these will draw power from the iPad - whose ability to provide power is very limited.
When using the recommended Lightning to USB3 Camera Adapter, the adapter is powered by the iPad Power Adapter - not the iPad. In addition to solving the power issue, you benefit from much higher speed data transfer. As such, when copying files/pictures from your SD Card, the transfer occurs in a fraction of the time. In practical terms, USB3 can transfer data 5-10 times faster than USB 2.0.
For maximum compatibility and flexibility, you need the Lightning to USB 3 Camera Adapter
https://store.apple.com/uk/xc/product/MK0W2ZM/A
You do not want the older (USB 2.0) Lightning to USB Camera Adapter:
https://store.apple.com/uk/xc/product/MD821ZM/A
If you just want an SD Card Reader - and forgo the flexibility of USB - you might choose the most recent version of Apple’s Lightning to SD Card Camera Reader:
https://store.apple.com/uk/xc/product/MJYT2ZM/A