Can any body tell me for what we use the big USB slot down  in the RC?

You can connect your mobile device there. Very practical for e.g. Crystalsky or when you use a tablet holder. You cannot charge the RC with this port.

The full size USB socket on the bottom is for the same purpose as the little mico-USB socket on the side. If you need/want to plug in your phone/tablet with a full size USB cable then you can connect to that socket.

I prefer to use that as I have had issues with the socket in the side with the included short cable so I stick to the bottom one.

Hi there. Thanks for reaching out DJI Forum. I just want to clarify, are you talking about the USB Port that is located on the center part of your RC? If yes, that is the USB/Assistant Video Link Port. You can use this to connect your device for video downlink via standard USB cable. You can also check this on Mavic Air's manual on our website. You may use this link and look for the PDF file of the Manual and download it. https://www.dji.com/mavic-air/info#downloads. Thank you.

If you decide to use the USB port on the bottom of the remote controller, make sure you remove the cable from the port on the left side first.

Hi, I would like to add information in case that you'll be using the other USB port like Msinger mentioned, ensure that the other cable is disconnected to avoid confusion to our RC on which USB port are we using. Please keep us posted for further assistance. Thank you!

RTFM