PDCP is also one of the LTE protocol along with others which provides internet services to end users through LTE network.
Since LTE is telecommunication technology and mobility is involved, it makes sure UE would be able to access internet under the defined mobility speed as per standard.
PDCP is also a protocol which works along with different protocols to deliver end to end services internet or internet based services. If I consider the example of downlink at eNodeB, PDCP is the layer which receives packets in the the form o GTP packet and remove the GTP header and make sure further packets will be delivered to UE in sequence by using different mechanism. All the coming packets get buffered at the PDCP layer and one sequence number is added so that when packet receives at UE's PDCP it can deliver packets in uplink in the same sequence it was received at eNodeB's PDCP layer.
In case of re-establishment when UE's radio link get broken and re-established again, it might be possible few packets were delivered to UE but it was not reached at UE. So just to synchronize both end of PDCP (eNodeB and UE ) PDCP status reporting procedure is used. There might be cases when same PDCP sequence packet receives. In this case PDCP will discard the duplicate packets. This happens when lower layers goes out of sync due to the re-establishment.