The most obvious reason for cutting 8c has already been mentioned: only two tiles are backfires if we cut it. The natural progression of the hand will be to remove one of the three 4b in order to make two sequences in bamboo, and then remove the second 8c so that we can wait on [4-6]c which includes the dora. In that sense, cutting 4b is not a bad choice either. Keeping it for now buys us the possibility of b if draw 7b, although that removes any possibility of pinfu. Also, keeping the [4-6-8]c shape gives us the possibility of declaring riichi while discarding 4c and waiting on 7c if that seems prudent based on the habits of the other players.