Dim-dip,as it was known back in the day,was a UK legal requirement.
Started in '85,I think.Certainly the D plate Nissan Vanette(

)I bought new was equipped.
The aim was to stop drivers from using just sidelights(they started to these'parking lights' at the same time)whilst the vehicle was moving.
Bear in mind that before alternators became a standard fit,it was considered normal practice to turn off one's headlights upon entering a 'built up area'(another term that dim-dip abolished!)to conserve battery power.
The elderly,people my age and older(!),find it difficult to see if a vehicle displaying just parking lights is moving or stationary.
A dynamo was not able to put out enough 'powa' to keep headlights.wipers etc running constantly.
This is just one of the issues you face when running a pre 70's classic,do you keep the dynamo for 'originality',or install an alternator for 'road safety' reasons?
Youth is wasted on the young.