What you’re doing is fine and has no quality loss. You shoot to cards and transfer the data over to the comp. via USB, that is correct and fine.
I/O cards by BMD and AJA and others, are used for Tape based workflows among others.
Believe it or not we used to shoot HD video to TAPE.
Then you would capture the media off the tape thru your I/O card onto the comp. Or if you have to lay out to tape a program for distribution, broadcast, duplication, Mastering or whatever.
Some shoot to cards, edit and lay out to tape. I/O cards also feed monitors for editorial, clients, color correction.
Chris