TCP spoofing is used to conduct SYN flooding.
Usually people think that it's possible to use TCP spoofing to hide the packet source, while creating a full running TCP connections, but to their dismay it's not the way.
Suppose computer A sends a spoofed packet to Computer B (SYN packet), computer B replys with SYN+ACK (if it listens on that port), but it will send it to the spoofed address.
Computer A will never receive that packet.
If you want to masquerade your IP the best ways are to use a proxy or a TCP relay.