"Heat Riser", typical on most 'street' type heads. It warms up the intake using exhaust gas cross over through the intake. In some cases coolant may be substituted for exhaust flow, not sure which on a 289. Keeps the carb from freezing up in winter and helps atomize the raw fuel suspended in the air/fuel mixture passing through the head to the combustion chambers, among other things. From a pure performance standpoint it 'upsets' the exhaust flow and 'warms up' the air/fuel charge. COLD air is denser, contains more oxygen thus more potential power. That could be offset by not enough 'heat' to get good atomization of the fuel suspended in the air. Running it promotes better gas mileage at a loss of horse power. There is ALWAYS a 'trade off' when designing an engine. Max power? Drivability? Best mpg? You can't have it all...
High Performance heads/intake will often block this hole using intake gaskets or the hole may not even exist. Like my OEM Ford High Riser heads, no provision for heating the intake at all. Most Edlebrock heads delete the cross over.