what is the use of the USB A port on the back on the smart controller?

The USB-A connector can be used for a 4G dongle, it can also be used for USB SD card readers, it won’t recognise USB  SSD but I suspect this is more to do with the size limitation of 128gb max card size. Which is handy if you have increased the internal storage with a card in the slot on the controller.
You can also connect your smart phone and share its picture library with the explorer on the controller.

A connected smart device will charge from the SRC.

The Go4 App on your smart phone won't connect to the controller, so you cant use your smart phone as a backup to the SRC display.
Edit .. You can use an SSD if its the right format! MSDOS - FAT32 - not formatted on a Mac, it doesnt seem to like that.

Welcome to the forum mate!

Buzzyone Posted at 1-22 23:25
The USB-A connector can be used for a 4G dongle, it can also be used for USB SD card readers, it won’t recognise USB  SSD but I suspect this is more to do with the size limitation of 128gb max card size. Which is handy if you have increased the internal storage with a card in the slot on the controller.
You can also connect your smart phone and share its picture library with the explorer on the controller.

Very informative answer. TY!

You can use it for pulling info from SRC , if you for instance have a multiple card types you can use a usb hub to download to different card sizes, you can also hook up a 4g adapter if you need internet.

Hi, thanks for the inquiry. The USB-A port uses to external devices, however, we would like to have a friendly reminder that DJI does not ensure that the DJI Smart Controller is compatible with 4G dongles and using a 4G dongle with the DJI Smart Controller is not recommended. Thank you for understanding.

Buzzyone Posted at 2019-1-22 23:25
The USB-A connector can be used for a 4G dongle, it can also be used for USB SD card readers, it won’t recognise USB  SSD but I suspect this is more to do with the size limitation of 128gb max card size. Which is handy if you have increased the internal storage with a card in the slot on the controller.
You can also connect your smart phone and share its picture library with the explorer on the controller.

I can connect my 1 terabyte ssd dongle to the USB A port only downside is speed because USB A is slow.  
USB C port is only for charging Smart Controller.

I suggest formatting ssd's or any external drive for this use with exfat file system.
Fat 32 has a 4 gig maximum on file size and is older technology. Any 4k 60 fps or any quality video will exceed this.
NTFS file format is not recognized and is only usable via windows operating system.
Ex fat file format is accepted by Mac windows Linux android, very universal without file size limits.

If you use a ssd / thumb drive after each flight to back up the slow speed isn't that bad.  However when moving a ton of data the best way is to invest in a card reader to USB C gen 2 preferably plugged into a laptop supporting like speeds of USB C.