They control the height of the cutterhead to which your blades are
mounted. Your blades need to be mounted the same distance from the
cutterhead. That is, they should extend the same distance from the
cutterhead to the end of the blade. This gives them consistent height
and hopefully makes them parallel to the table. If the blades are
mounted consistently then you could use those screws to move the
cutterhead up/down and make it parallel to the table. If you're blades
are not consistently positioned then one blade will contact the wood
and another one won't.