What does this actually do? I see intake gaskets that block this off on the heads. Good to do? No? Why? Thanks!

Keeps the heat out of the intake but hurts the cold weather manners.
Will make cool / cold season warm-ups take longer. Just have to feather the throttle till it warms up.

You'll need the gaskets with crossovers blocked if you intend to use a raised plenum single or dual plane manifold. i.e RPM Air-Gap.

It also is how the choke spring is heated up so that the choke will open. This only applies to non electric chokes. It is designed to channel hot exhaust gas under the carb and heat the intake for cool weather performace. It only works well with the stock exhaust manifolds and the heat riser installed on the right side exhaust manifold.