请问为什么PDCP下行数据传输时，Received PDCP SN已经出窗，但却不直接丢弃，而是先解压缩解密后再丢弃。

if received PDCP SN – Last_Submitted_PDCP_RX_SN > Reordering_Window or 0 <= Last_Submitted_PDCP_RX_SN – received PDCP SN < Reordering_Window:
    -     if received PDCP SN > Next_PDCP_RX_SN:
        -     decipher the PDCP PDU as specified in the subclause 5.6, using COUNT based on RX_HFN - 1 and the received PDCP SN;
    -     else:
        -     decipher the PDCP PDU as specified in the subclause 5.6, using COUNT based on RX_HFN and the received PDCP SN;
-     perform header decompression (if configured) as specified in the subclause 5.5.5;
-     discard this PDCP SDU;

协议里写的是解密后无条件丢弃，既然要无条件丢弃，还要解密做什么？

不把数据解出来，如何知道该数据是否有用那？