Vacuum line

[BlackBeast]

Slowly planning out my catch can install, I need to know, the vacuum line running from the inlet manifold (that little annoying elbow by the powersteering pump) to the heater valve on the intake pipe, what does it actually do? Whats its purpose?

What would happen if I blanked it off at both ends?

[woz]

...What would happen if I blanked it off at both ends?
Dont know what it does but dont forget its vac and +ve pressure, when blanking it off;>)