There are two types of altimeter sensors: Barometry and Accelerometry. Pure accelerometer based altimeters do not require ports to operate but they are rarer. Any altimeter based on barometric sensors will require a vent port to the outside to measure changes in pressure. These air vent holes allow the movement of air between the inside compartment that contains the sensors.

There are two keys to making vent ports: the ports should be placed so that they do not receive turbulence during flight and their combined size must be appropriate for the diameter of the rocket and altimeter. Turbulence can increase if you put them on the slanted sides of the nose cone or directly across the tube from one another (I personally think the latter is notional).