I have a few guesses here:
1.) A cutoff valve you can manually use when doing maintenance? seems unlikely tough.
2.) A one-way valve which prevents fuel to flow back to the tank when the car is turned off.
3.) An oil pressure switch. Some cars have an oil pressure switch which cuts the fuel if the pressure drops too much. I recall this was found on some US cars rather than japanese, but cant say for sure. The tread at the bottom would indicate a pressure switch at least. Try blowing in it a few times to see which way it flows =)
EDIT: I am actually betting on #3. as "A valve cut" ?=> "auxillary cut-off valve"?
To anwer your question: Dont fit it if you dont know what it's for. You could just cause more trouble if you happen to install it the wrong way around. Just remember to prime your fuel pump before starting the car in case the part is a one-way valve to prevent fuel flowing back to the tank.
-Willem