Radiator overflow hose/nipple - what's it for?

[Dave C]

On the type 44 radiator ... what is the purpose of the overflow hose
connected from the top of the radiator to the overflow tank?

My little plastic nipple broke off.  While I'm trying to think of a way to
repair it I have to wonder why can't I just plug it up?  I think the
overflow setup is similar on the 4kq as far as the tank and pressure cap.
But instead of the overflow hose the 4kq has only a vent with a screw in it
... used to vent air from the system.  I think my radiator is solid aside
from this broken nippleotherwise... I just wiggled a bit too much on this
little plastic thingy while flushing the coolant and messing with the hose,
etc.

If this overflow hose is actually important/useful could someone suggest a
way to repair it?  I've not heard of a kit.  I'm thinking it might work to
splice a short piece of 1 1/4 inch OD pipe into the radiator hose with a
nipple attached to it somehow, and run the hose to it, then screw a plug
into the hole where the nipple used to be.  I haven't figured out which
types of pipe are available in the right dimensions or how to attach the
nipple.  If it turns out the overflow hose is unnecessary I'll be happy to
save myself the trouble.

thanks,
Dave C.