I suppose the purpose back in the day was to supplement the poor stock lighting, but certainly for rally/high speed driving. The yellow lenses were supposed to cut back on reflected glare from water vapor in the air because of the wavelenght of yellow light, but this theory has been disproven.
I have them wired to come on with the high beams, along with a switch to turn them off if I don't want them on at all. They light up everything ahead, especially signs, from very far away.
I rarely have them on since I do mostly city driving, like I said before, I just like the period look as an accessory. Like 14" Alpina wheels for example, which I'd love to have someday also.