>>wrboyc+(OP)
TCP packets contain sequence numbers that must correspond to the ones sent by the other side. This is an issue if you're spoofing packets because you don't receive packets (containing the sequence numbers) from the other side (they will go to the spoofed address, rather than yours). Without the other side's sequence numbers, your replies will be considered invalid, which means you can't complete the handshake[1] to establish a connection. However, if you can successfully guess the sequence numbers, you can complete the handshake and also write arbitrary data to the stream. You still won't be able to receive data, but for simple protocols like irc, it can still be useful eg. connecting to a server and then sending spam to an user/channel.
[1] https://en.wikipedia.org/wiki/Transmission_Control_Protocol#...