white plastic thing On top of the fuel filter

[Golf/Jetta]

what is the white plastic thing on top of the fuel filter? (IP return - filter - return line to tank )
1997 Jetta 1.9L TDi half-breed
what does it do?
and when is it recommended to replace it? (cracks, defective)
how do i know?

[Dr. Diesel]

heater recirculation valve. kinda acts like a thermostat to regulate fuel temperature, using fuel warmed up from the pump.
new filters should have fresh sealing O-rings on them. It's cheap enough to change if you suspect it's misbehaving in any way.