First, to make sure that if they ask you, you have it. It is a basic and quite obvious statement, but with so much opinion going around, it is worth remembering. Currently, it is the only way to prove the validity of your National Driver's License in the countries attached to international automotive regulation conventions.
Second, because it is a translation into several languages, in a format known and admitted internationally
Third, because it is required in case the tourist is involved in traffic problems, whether they are infractions or accidents outside the country.
Fourth, because depending on the insurance company, it is required to cover damage to the vehicle in case of accidents outside the country.
Fifth, it allows to prove the tourist status of a person without the need to show the passport, this only before the road transport authorities.
Even if you are riding a shared minibus, the person you drive must meet the requirements for driving in Europe