Why would you want hot, less dense air going to the cylinders? And at the speed at which the air is traveling to the cylinders (the single barrel carb can do 300cfm) you are not putting much heat into the air.
Page 6-5
http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm0605.html describes the operation of the riser.. its clearly only used at one time: startup.
I can't see how this does anything but as mentioned above... vaporize excess fuel that may pool at the bottom of the intake due to the choke or 'pumping' the gas pedal.
I've looked for a detailed explanation of why its there... haven't found one.
