Given two or more readers that are connected to the DSMR. Both readers have their own data-request line, but the data output is shared.
If the secondary port starts reading, by toggling the read request, while the primary is still busy, the secondary will read an incomplete and invalid telegram. Furthermore, it has to wait ~1 second before the second telegram is outputted by the DSMR.
If the secondary port did not have the data-request line connected, it would not been able to read a complete and valid telegram.