[Spiderworks]

Can anyone explain why its needed?

Earthing / bonding cables are often used in petroleum fuelling systems ( big and small) to equalise electrical potential. If all components in an fuel handling system have the same electrical potential, then there is no possibility of static electricity being generated to the point that a spark can be produced. (we all know that static electricity generates high voltages and produces sparks right?)
Sparks from static are very bad when a fuel air mixture is present. (such as in a carburettor).
All fuels moving through fuel lines generate static and this static needs to be managed. Static is a VERY big problem at refineries and in aircraft fuel systems.
I've seen lots of photos of alfa engine fires. I would strongly suspect the little carb earthing strap was either removed or not working as designed and caused many of these fires.

 

[Spiderworks]

I'm not familiar with the pollution device, but yes fuel fires are not uncommon... I havent seen the actual fires, but a number of photos Ive seen after the event (such as this one) seem to start in the right hand side of the engine bay. May be static, may be something else. I'm sad for the car. No doubt the fireys are looking for rust in the spare wheel well to see if she if she is a candidate for a rebuild.