Angle Milling

Ok so i have some questions in reference to angle milling heads. Lets go back to a time when all these aftermarket heads were not available with extra meat and new valve angles over stock. What was to gain by angle milling heads vs just a straight cut? What work was involved after the cut and was it work it for what was available? Lastly on our wedges what is a good valve angle, is it a less than kinda number (closer to 0* the better) or is there a range?

thanks!