In SR$,$ why does the receiver have to acknowledge packets with sequence numbers that are less than $($and to the left of$)$ those in its window, which starts at rcv$\_$base.

Because the sender may not have received an ACK for that packet yet.

Because, at the time of the data packet arrival at the receiver, the sender has definitely still not received an ACK for that packet.

Actually, this ACK retransmission can be ignored and the protocol will still function correctly, but its performance won$$t be as good.