TCP: FIN_WAIT1 to TIME_WAIT

Hi,

I have the following question (purely from an academic standpoint)
regarding the TCP state machine diagram (as is given in books authored
by Richard Stevans, Comer etc. ). There is a transition from the state
FIN_WAIT1 to the state TIME_WAIT upon receipt of a packet with both
FIN & ACK flags set. In what scenario(s) will the other end send such
a packet ?
[ I was trying to figure out the scenario from the 2.6 source code. I
was able to find the code corresponding to the other two transitions
only viz "FIN_WAIT1 to FIN_WAIT2" and "FIN_WAIT1 to CLOSING"]

Thanks for your reply.
~
Regards,
Radha