I am aware of other numbers being used in such cases. Think company registration numbers or LEI numbers.
Basically it is a check on whether it is correct to not charge VAT.
In one of my previous companies we were forced to have a VAT number (canton of Zug).
One of the things that changed︀ at one point was the threshold amount. It got significantly lowered, I think because of︁ EU harmonization? Shortly after we had a VAT number..
Added to that, it doesn’t always︂ have to be your home country requiring you to have a VAT number. It can︃ also be your clients home country.