A B-CAS card is necessary for Japan if you want to watch ISDB-T in 1080P. If don’t insert the b-cas card in the ISDB-T receiver, then the tv picture is 720p, for the small screens, this picture resolution is acceptable except that your in-car monitor is HDMI input.
Except for Japan, other countries’ ISDB-T doesn’t need the B-cas card.

B-cas card reader for Japan ISDB-T