lets you have a front wheel ‘cutout’
frees up head tube shaping options, and that is the most important bit on the w hole bike
top tubes do still add drag, even on a velodrome, yaw isn’t 0 the whole time there either
These were the USA Team Olympic track bikes ('92 I think). I never could understand the design; if you were going to save drag by omitting one tube, why remove the top tube?